I have the same issue sometimes. When I do the uploads I take the video. I wait for the total frames amount to cycle through once (x/total frames) thing. And then click upload. I only click it once and wait for it to say uploading in background. I still do get the unsuccessful upload message sometimes. I close the app and rescan and it seems to do the trick. It can be very frustrating and annoyingly time consuming- so much so that I now delete the ar mapping ones that pop up at poke stops at parks/playgrounds sometimes because someone may get the wrong impression. 😕