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It sounds like you are using far to big a brush for refine edge in select and mask.
Use the quick selection tool initially to get a better selection then keep the brush small (10-20px and run round the edges only with the refine edge tool
The most difficult images to accurately select are those that are not clearly defined -- slightly out of focus. That is what you are dealing with here. In addition, some of the hair strands are white and would be lost in silhouetting. I suggest that for starters, if you want to practice the technique, you begin with a less challenging subject; a sharper one with greater subject to background contrast.
Regarding your actual file (rather than your screen grab): it is possible that a technique other than refine edge (such as a modified Lab or other channel mask, or Calculations and an appropriate final background would be a better option. Refine Edge is not a one-method-fits-all device.
I've been having similar issues. I tried all of your suggestions. I made the refine edge brush smaller (I've tried 20, 15, and 10 pixels per your suggestions, as well as 1, 3, 6, and 7 pixels, with hardnesses of 100%, 80%, and 60%) and carefully painting only along the edges, as well as only within the bounds of the edge feathers that weren't yet selected. I still found that it did an unsatisfactory job. For example, I was trying to do a background replacement on a photo of a bald eagle photo where the background is washed out to the point where it's white, and there is nothing in the nothing else in the image other than what I want to mask and the white background, yet it still had a hard time, even with the brown feathers. How much more contrast can you get? The image is quite sharp, so being out of focus isn't the issue. I've watched plenty of tutorials on masking hair, but I my results are not nearly as good as theirs. I've started with both 'select subject' and the lasso tool, taking care to get close to the edge, touching it up with the regular paintbrush, but not to overshoot it. Also, how will I know if my brush size is too small? Will it just not add to the mask, without cutting anything out?
I'm using CC 2018 (latest version), and I haven't found anything that suggest that this version has a bug that might cause this lackluster performance. To give you a better idea for what I'm trying to mask/in case my assessments of sharpness and focus are off, I'm attaching a slightly lower resolution jpeg of the image I'm talking about (60 quality, optimized for web). But if it's relevant, I've tried working on both the original RAW file and the 100% quality jpeg, not this lower quality version. I've also tried playing with edge detection (with and without smart radius), as well as boosting contrast and shifting the edge in the global refinements sliders. I've also tried leaving the sliders alone. I don't want to add too much contrast in the actual output file due to noise concerns (I have a micro-4/3 camera and this was at dusk in slightly windy conditions), so that approach is not ideal for me (unless it doesn't alter the contrast of the image, itself – just the mask). Any suggestions? I've also tried the background eraser tool, suggested in some tutorials (setting and protecting foreground and setting background), with worse results after countless hours of experimenting with it, but that is getting off-topic. I feel like this image should be a pretty good candidate for background replacement. I've seen detailed selections done in post-processing on images that are far more complex, with excellent results. Thanks!
Is there a better technique for doing a background replacement for the image I've attached? Perhaps a channel mask? I went into more detail about my issues in my reply to davescm, above (in case you want to know). I also briefly mentioned that I tried the background eraser tool, as suggested in some tutorials (setting and protecting foreground and setting background), with worse results than select and mask, even after countless hours of experimenting with it. When you say a modified lab, do you mean infrared or an alternate color space? Or do you mean a 3rd party plugin for masking? Or is there a tool called modified lab? A search under help for 'modified lab' turns up nothing, so I'm assuming not.
To my eyes, my image looks fairly sharp and in focus, but I could be wrong and I didn't sharpen the RAW file much due to noise concerns (shot on micro-4/3 camera at dusk, windy, handheld – no time to set up a tripod before it flew away). The focus isn't 100% perfect throughout (400mm focal length and this was as close as I could get to my subject). I also don't want to boost contrast too much, per the noise concerns I've mentioned above. I can understand it struggling with the whitish/gray feathers against the white background, but it even struggled with the brown feathers. I have yet to further remove chromatic aberrations in photoshop – only did very slight removal in DxO photolab, but I can try that, too, prior to replacing the background, if that's hindering masking ability.
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For that particular image I would use a channel rather than Select and Mask :
1. Make a duplicate of the blue channel
2. Invert that channel
3. Use a curves adjustment to increase the contrast (particularly the black)
4. Draw a rough selection around the inside and fill with white
5. Take a soft round brush and set it to overlay blending mode and 50% opacity. Brush around the edges with whiote and black to lighten white and darken black - don't overdo it. The idea is to sharpen up the edges
6. Make a selection from the channel and then a layer mask from the selection
On a light colour that is it :
On a dark color you will need to darken the edge contamination slightly with an additional layer set to darken and painting with a darker colour picked up from just inside the wanted image.