Can a UPVC door warp?

I've recently had a new double glazed UPVC front door fitted, it is becoming increasingly more difficult to close it. If you push it to with anything less than a reasonably strong push it bounces back. I've checked it from the outside with the door pulled to but not closed into the frame & noticed that when the door is touching the door frame at the top & bottom there is a gap of between 3-5mm at the point where the door handle & lock are situated (middle of door). It is becomig increasingly necessary to push the door into the frame at the lock to turn the key in the lock. If it isn't a warp has it just been poorly fitted? We have no problems at all with our back door which was fitted by the same company about 5 years ago.