Probably not a lot you can do....but have you tried playing the card in the Action Cam ( one with a screen, or micro-USB port )...?
-This should be the best-chance for the files to be seen as "correct" - as I'd expect any writing software would check it had written correctly (or output a message ). Of course if there is no LCD screen ( as some GoPro are detachable to save Battery-power ), then I can't imagine how it tells you when the card is Full.
You said card was "lying around" - Do you have no System in place to protect these cards from static ( or children /pets swallowing them?)....In future use only cards that you can Copy a large chunk to....Check in "Properties" and even play Fist/Last Vid-files. ( E.G. from a block of earlier Vids about the same Capacity ).
...then Format in the Action-Cam.
- Did you omit this step, because of the Holiday Rush, perhaps?
FWIW my cards are ID as; A, B,C, etc. painted in nail-varnish ( discontinued colours are £1 ).... then in the PC / Master Copy / Size...... they are given a Number 1-, 2-, 3- etc. followed by the letter. ( So I know which card did the shoot, should I need to go to the "source" over the next few Months..).
By the time I get to number 20-B that card has proved trouble-free ( Or is that "Nearing Retirement"? ).
Others will have better Schemes . . . but after much confusion, this is what appears to work.
Thus, the card I gave as an example is: Drive Letter / Master Copy / 16G-20-B . . . to give it the full title -
The letter B on the card, also helps when on a shoot - if I need more capacity, then I'll Format the next SD card= "C" in the camera - Back home, I can see which card is First, etc.
Odd that only the first file was OK....did you not have Vids from earlier usage of the Action-Cam? Those would have shown a similar "NO-GO" symptom, surely? Is it possible the files aren't Movies..... -or- the Action Cam is faulty? e.g. water-seal
I know the Camera-Menu can be very confusing, but if a file is recorded, it should comply with the correct standards; even if you've set the resolution very low ( that dreaded "digital Zoom" ).
Your description reads like "card faulty".... but I hope the above may assist over time...
Good Luck.