At hobby lobby or michaels you can purchase crystals that are called hot fix and you can replace the ones that have fallen off with an iron. Just be careful not to damage the material when applying them. You can also buy a tool to appy them but then you are spending more money. However I have added crystals to leos that did not have any at all pretty cheap and it really changes the way the leo looks!!
If the stones are just glued on, you could try a hair-dryer or putting the leo in the freezer. Both methods have worked for me when I was trying to remove super-sticky price labels. Goo-be-gone stuff is normally a petroleum derivative and can be VERY stinky, so I'd be careful with chemicals.
I have had leos where the stones have fallen off and i mailed the leos back to GK (they were GK leos and I had paid a fortune for them!) and they replaced the stones and sent the leos back to me with no charge. One i even sent them the week before a meet (of course it was her competitive one) and told them I needed it back in hand the next Thursday and I got it back by then and never had another issue.