Just wanted to point out that for the first set you have the SKIP TAB instead of START overlay there, so you have SKIP TAB’s opacity twice. I know that isn’t the issue but I just thought I’d say it even though it doesn’t actually matter since they should both appear as opacity 0 anyway.
Did you make sure your overlays aren’t overlapping each other? Because the transparent areas of each overlay are important and count.
Edit: Just noticed Julie said prior. 
I can’t see anything wrong with your script, that’s why I’m guessing it could be the overlays. Are you also testing it properly by making sure to navigate back to the start of the scene before testing each button?
Don’t know if you’ve added anything below your bottom label (another scene, a transition, a pause, anything), but if you haven’t, I recommend that you do.