Could be faulty lock / micro switch . Could try this from Vectra forum .
" I had this a few weeks ago...
The car had been in for the tailgate fault, had to phone the dealership back on getting home
Raise the tailgate
Insert a screwdriver into the hole next to the lock and PUSH
Insert the key into ignition, turn to electrics on, press tailgate unlock button, remove key from ignition
The lock motor should now be heard (which it was)
Lower tailgate and close
If the tailgate does NOT lock the motor and/or electrics have failed, and to bring the car straight back
However, it worked for me, and the lock has worked correctly ever since "
Posted Jan 16, 2011 (13 years ago)