Is this with the engine running (or perhaps even the parking brake not set)? Often there's a lockout for a front TV set to prevent operation when driving for legal reasons. The laws vary slightly in detail from state to state, but in some any TV screen in front of the plane of the driver's seat back can't operate while driving. Others limit it to ones that are not in the driver's line of sight.
Displays dedicated to driving tasks like GPS or rear view cameras are excepted, if not specifically in the law then certainly by general practice.
