Try resetting it. Often those instructions can be found in your owners manual. These are generic instructions:
Sometimes the liftgate can lose it's reference points and it needs to be reset.
The power liftgate may not operate properly and may need to be reset if any of these conditions occur:
• A low voltage or dead battery
• Disconnected battery
• The liftgate is manually closed and left ajar (unlatched)
To reset the power liftgate:
1. Disconnect the battery for 20 seconds then, reconnect the battery.
2. Manually close and fully latch the liftgate.
3. Power open the liftgate by using the integrated keyhead transmitter, or the instrument panel control button.