The patio door that was a problem for us looked old and was in place when we moved into the house so we had no way of establishing the age, construction or make etc. We also couldn't figure out how to test what the problem was caused by - the room is adjacent to our kitchen but I was already cooking with saucepan lids on, using an extractor fan that vents externally to try to minimise moisture in the air. The lower corner of one side of the frame was really cold to touch when the outside temperature dropped and that's when water would bead up on the frame, eventually soaking into the carpet and curtain. But there was nothing that we could see inside or out that indicated any failure - no cracks etc.
The door was mentioned in the buyers survey as being a weakness because the runner was external and it only had one lock, so we always knew that it needed replacing. The fitters had a look at the old frame when they took it out but there was nothing obvious to the eye so we presume it must have been either the construction was always a poor design or something had failed inside that meant there was no thermal break as Soopercrip suggested.
So not much help really but I would strongly suggest that you take it up with your fitters Jadyo & Leon10 because in our experience it won't resolve and your problem may be covered by the warranty. Best of luck.