Why do some videos play fine on my Android, but not others?
Your Android device will play the file types supported by the device. The supported types will vary from one make/model to another.
If a file is not playing on your Android, upload it to your RealCloud and then play it from there. RealCloud will automatically convert it to a format supported by your device.
If you want to play it locally, you can download the file from the Cloud.