Why will some mp4 files play on my samsung galaxy s5 and some wont?

[AC Question]

Ok so i'm having a major issue trying to get videos to play on my phone to use with my VR headset. I have Iskysoft video converter ultimate that I use from a windows 7 pc to convert .avi files into the s5's format of mp4. I recently purchased a cheaper 64gb sd card (unsure of what class) and on that card after they've been converted and copied over the video's have a playback error. I've tried inserting the SD card and copying over the files directly from the computer, but no luck either. I formatted the card to the phone last night, and thought that would be the last of it, as the first 10 files I copied over worked on the phone, after that they were coming up with the same error.(also the thumbnails on the phone don't show a preview of the file, they come up with a blank icon of the player). Is it the SD card that's causing the issue? The computer? Are there certain classes of card I should be using with the phone? I have an old Huawei Y530, and it plays everything with no dramas. It's all very confusing, please help!