Went to a customer recently whose UPVC door wouldn't open when the handles were pulled down. After I opened the door the lock seemed to be working perfectly. The problem was that the door was so low on one side that the shoot bolt at the bottom of the lock was sticking in the keep. After adjusting the door it worked fine. In a strange coincidence the following day I went to look at a window that wouldn't open and this was caused by exactly the same problem. Both of these problems would have been much more expensive to fix if left until something actually broke, so it makes sense to get sticking doors and windows looked at sooner rather than later. We don't charge to come and give a quote so you don't have anything to lose by calling us.