A couple of things to check:
Confirm you have 1-2Gb free in internal storage (if you already did this skip to the next item).
Confirm all videos you are using were recorded directly with your device's camera app (not other apps) and are stored on your device's internal storage before you start making your collage
Confirm all your videos are normal HD (not 4K/UHD) and normal play speed (not slow/fastmo)
Confirm there are no video and camera apps running in the background (if some apps are running, close them and power off your device for 30 seconds and then restart)
Go to Android Settings > Apps > PicPlayPost > Permissions and confirm you have allowed permissions for Camera and Storage.
If you have a song added, try turning it off.
Confirm the song you are using is an mp3 file (not an mp4, m4a, or other file type).
If you are using gifs in your collage, try changing your frame to a single (1 box) frame.
If you have "Play videos sequentially" on, try turning it off.
If the issue persists, press the gear icon in the app and use the "Email us a Question" option to tell us what media you have in the frame - photos, videos, gifs, music.
Please also send us all the media you added to the frame and a screenshot of your layout so we can investigate - if the files are too large to email, please upload them to Google Drive and share with: support@mixcord.co