There could be two reasons.
1. You may have your schedule enabled via the iOS or Android App. When the schedule is enabled, Opal will check to see if it should be on or off once every hour. If you manually turn your Opal on, but it is scheduled to be off, it will shut off the next time it checks the schedule. This will usually occur at the top of the hour. To fix this issue, go to your app and disable the schedule.
2. Your ice bin may not be pushed in all the way or not installed in your Opal. The ice bin must be fully inserted or Opal will turn itself off after approximately 5 minutes. Another way to disable the schedule is to unplug your Opal, wait a few seconds, then plug it back in.