Sometimes a downloaded file can get corrupted (damaged) on your device. If this happens, you might see a "Video Load Error" or "Content Load Error" when your loop tries to play. Clearing the VMcast app cache on your Amazon Fire TV Stick may help.
From the VMcast app, click down on the Amazon Fire TV Stick remote to highlight the gear (⚙) icon.
Select "Settings."
Highlight and select "Empty Cache."
A notification will appear once all of the cached files have been removed from the device. After this message appears, select and play your loop. The device will re-download your cacheable content.
If you still experience playback issues, you may need to clear all of the VMcast app preferences.