I can confirm that it works as you described in the newest version of Photoshop. There are several things you could try.
Edit > Keyboard Shortcuts. Make sure that keyboard shortcuts are set to defaults.
If that doesn't work, reset preferences. Preferences in Photoshop
If that doesn't work, use the CC Cleaner tool. Use the Creative Cloud Cleaner Tool to solve installation problems
Are the layers native to Photoshop or have they been placed into Photoshop?
Could be that the content could be Place Linked, where the linked file is accessible at work but not at home.
The layers are native to PS. I've created them with PS brushes so they're not placed content.
I tried turning off the processor. It made a slight difference but it hasn't fixed the problem. I now can get the little plus however not every time, it's glitchy and not behaving smoothly or consistently.
I also reset my shortcuts as I had modified them but that didn't work.
When I spoke to Adobe support they reset my PS pref's but it messed up all my brushes, and PS stopped remembering my custom prefs each time I shut down. I had to customise all over again when I opened PS. I eventually had to ring them back and they put the pref's back. I'm working to a very tight deadline of Aug 28th so I think I'll have to put up with the fault until then. Then I will try resetting prefs and if necessary using the CC Cleaner Tool as you suggested.
I tried to recreate your problem on my end unsuccessfully.
Do you think you could pop through a short recording of your experience so we can get our heads around this?
Sure. Can you recommend a program I can use to record? I've never done it before!
I've recorded using Quicktime... just need to figure out how to upload it. Sorry!
Here is a recording. Password: Pixelproblem
In the video I'm first selecting pixels in a layer by command clicking on the layer thumbnail. The black section of whale is selected.
I then attempt to select and add pixels from a different layer by command shift clicking on the layer thumbnail. A plus sign should appear allowing me to do this. However, you can see the plus sign flashing on and off sporadically. When I manage to get it to stay instead of selecting the layer pixels it just highlights the whole layer in the layers palette.
My aim is to have the black part of the whale and the green part of the rubber ring all selected together. This should be easily done using the above commands.
I hope that makes sense. It's very hard to show/explain.
Just to update I have today uninstalled and reinstalled photoshop and I'm still experiencing the same problem! If anyone can help I"d greatly appreciate it.