Flash drive not showings some files but showing most

I recently bought a 512gb flash drive and started using it to copy movies onto from my computer. Most movies I try to copy work and show up but sometimes the folder will copy but the mp4 file inside wont show up. When I try to re-copy the mp4 file alone a pop up says “a file of said-name already exists in that place” then gives me the option to stop, keep both, or replace. But the movie still wont show up