Had random problem occasionally over past few years where power tailgate would close, appear to catch but slightly pop a few seconds later, the tailgate could then just be lifted open. 
Couple of days ago the problem happened again and has remained, so I decided to strip it down. Found abit of a design flaw which while not permanently fixed it can be easily be reset to work again. 
Remove the  plastic cover on catch secured by 3 Philips screws

once open you will find a motor driving a long worm gear, on the there will be a rubber sleeve and a spring.

the rubber sleeve moves along the gear and gets jammed preventing the spring working and jamming the catch open.

just slide the rubber sleeve back away from the spring towards a section of the worm gear that is smooth

it should then work again

Glad you sorted it, well done for the guide as I'm sure it'll help others.