had a problem again with my wifes GD tailgate, it was unlocking but wouldn't open (i could hear and feel the solenoid working when i touched the door handle.
I lowered the back seat and took out the little round plastic bung just above the lock, last time this happened the little lever inside was stuck in the 11 o'clock position and pressing the key fob did not move it, you can push it back to 1 oclock position with your finger and watch it as you press the keyfob (all car doors need to be closed or central will not try to lock doors) , when pressing the 'open' button it should move across to 2 oclock position showing the solenoid is working.
even when this was happening, the normal handle trigger didn't want to open it.
after much time spent on t'internet it seem this is pretty much a problem with every car on the planet, but didn't seem to be too many answers.
what i did find out is that unless the opening handle flap above number plate is in its fully forward position - PUSHED RIGHT FORWARD towards the tailgate when you press the unlock button on keyfob (or the key in the door-lock) the latch somehow becomes disconnected and the handle doesn't work.
I pushed the opening handle fully forward and pressed the 'open' keyfob button, and the handle now works, just to check I held it away from the tailgate and it won't open again, I pushed it forward and it worked OK. I will now lubricate the pivot points of the handle to try and make it always return to its fully forward position when it is released.
NOTE - all the pushing and pulling in the world will not release the tailgate if it doesn't want to open, and trying to move the lock tongue back with a screwdriver doesn't work either .
EDIT - found the problem, the plastic release handle above number plate is a tight fit in its steel bracket, the bracket had rust on inside cheeks and this and was preventing the return spring from pulling the plastic handle back to its home position.