I assume you do mean "opening" and not unlocking. I had something similar on my T5. From what I understood there is a lever on the opening mech that needs to be "cocked" in order for the door handle to open the door. This lever can get stuck or become sticky. If it is not in the "cocked" position the door will not open. If stuck you will need to remove the door card to get to the mech and see what the problem is. There is a small spring next to the lever that gets in the way of the lever if the spring moves. If just sticky, I found a good squirt of WD40 works. Alternatively a visit to VW.