Button Opacity Animation Not Working

Hi there, I have a simple space shooting game and I am currently working on a page where the player selects a ship and a difficulty. I have animations on some of the buttons and most are working fine. However, on the difficulty buttons, once clicked, they either flash on and off (its meant to be a faint flashing not an on and off) or they just completely disappear. Also, once one of those difficulty buttons disappear, any other button that get hovered over, also disappears...I'm not actually sure what the problem is but I'm pretty sure its a problem with the animation. So for that reason I haven't posted any code as I don't think thats part of the problem. Anyway, below is a couple of screenshots so you get more visual idea of whats going on. Any ideas on how to fix it?

Here is the menu:

And here is a screenshot of the problem, you can see that the 'expert' button has been pressed, and most of the ships have gone, the slider button has gone and expert button itself has gone too...