A Bucket Fill of a transparent area fills only pixels that are 100% transparent. Some of those near the lines you've painted are actually partially transparent - hence the fill stops before reaching the completely opaque pixels.
If you want to do filled drawings, consider drawing paths and using filled shapes as an alternative. Or you could select the transparent areas with the Magic Wand, expand the selection a bit, then paint in the selected area with a big brush..
Oh, that makes sense! I did simply stroke the path... when I could have filled the path instead.
If that option doesn't always work, your suggestion to select, and expand also has it's place.
Thank you for the tips!