There are a number of possible reasons:
1. It could be a faulty or counterfeit card (especially if it's a generic brand, as you say). What happens when you plug the card into your computer? Does it say that it has 64 GB capacity? By the way, always make sure to
Unmount the card before removing it from your phone. You can Unmount in the
Settings>Storage menu.
2. I'm not sure if the S4 supports AVI natively--maybe someone else can comment on this. If it doesn't, you'd need a 3rd party player like
MX Player.
3. There are lots and lots and lots and lots of threads about difficulties with external SD cards in Samsung devices, so ultimately, this might be the problem. Android itself actually doesn't have native support for external SD cards--the code that's used is a workaround. Sometimes that workaround doesn't work too well.