PVC back door won't lock?

Hi, can't turn the key to lock the door. When I raise the handle all four bolts go up into the correct position but still can't turn the key. I have replaced the cylinder with a new one, but still no joy. Also, have tried removing the brass strip that runs the length of the door, but am unable to remove the bolts to release the strip. An allen key seems to fit ok but the bolts won't unscrew and just keep turning in the door.

Help much appreciated.