I went through this myself a few years ago.

The vacuum actuator for the tailgate is different than the doors. The part number from the EPC is A 123 800 24 75.

My latch assembly was not very free moving and still isn't the best. Anything less than an optimal vacuum to the element would cause it not to cycle fully.

The big culprit in my case was the fuel door element. The plastic case was slightly bulged causing intermittent leak downs. I replaced it with a better used element and am getting more consistent lock/unlock action.

As noted in the other answers lubrication is important.