I just bought a 32GB USB stick to store 15.7GB of video on. When I get about halfway, any more video files I add will not play, or in most cases will not even show up. I've tried reformatting a few times.

Which filesystem are you using?

You can also do a quick check with diskmgmt.msc, or, if you are willing to do a little reading you can use diskpart.exe.

If you use it to transfer data between Mac and PC, then it is FAT formatted, and FAT only supports files up to 4 GB.

Testing takes time, but one simple test can find a lot of problems like fake sticks. Just write the three byte pattern 0x55AAFF over and over, and then verify it. If it is a fake stick, the 3 byte pattern will go wrong. To speed writing an checking, you can extend the 3 byte block to 24 bytes so 8 byte writes can be used.

If that works, do again with a 1 and then 2 byte stagger in the pattern, so all bits are tested both ways with adjacent bits opposed.

Writing and reading a 32G stick 3 times takes quite a while. Frauds depend on this to mask their faking!