Sometimes the videos do not play well and the device gets stuck in a loop (playing the same videos that don't work). Best case scenario is you get on the good side of a loop.
You can either keep working through it (which often times is not worth it). When it goes black, keep back spacing and or restarting the app as needed until you find success. You can also try clearing your cache and data in the app settings.
You can wait for another block of videos to run (which means you are not getting credits while you wait), but may be your only strategy. Trying a VPN here may help you out a lot.
You can switch devices. Between a tablet, phones, and an android emulator on the computer. You can usually get at least one if not all to work. Don't be afraid to combine strategies. Good Luck.