There are a number of reasons why a uPVC or composite door wIll not lock, due to the ever increasingly sophisticated locking systems being used the enhanced security has increased the risk of this type of doors to suddenly become difficult to lock, sadly this is a very common issue. 

A door that has either dropped or become out of alignment can cause the locking system to foul the keeps and not engage correctly, if this is the case a uPVC door will require "Toe and Heeling" or adjustments to correctly align the door leaf with the frame, poor installation is often the root cause and some doors will remain problematic if an installation issue is not rectified.

Sometimes the issue is failure of locking system, Euro cylinder or the locking keep securing screws. 

Extremely hot weather can temporarily affect the operation of a uPVC door by causing it to swell, this is even more common with coloured upvc doors, in this instance shade the door from the Sun and allow the door to cool down, this usually resolves the issue.

Composite doors are commonly affected by the weather, certain doors, especially if South facing, can be prone to bowing, a door that has a dark colour externally, an Anthracite Grey door as an example, will absorb more heat than a light coloured door.

A bowed composite door may be subject to replacement under the guarantee, generally there will be a tolerance specified by the manufacturer, this is usually around the 6mm or 7mm mark, this can be checked by using a 6ft level or straight edge. Keeping a door fully doubled locked will reduce the risk of bowing. It is common for doors to straighten as the weather conditions change.

Composite doors have tighter tolerances than a uPVC door, poor installation will generally result in a door that is difficult to operate which is usually exasperated during periods of extreme weather.

It doesn’t matter how good quality the product is,  a poorly installed door will generally be problematic with difficulties with locking and operation. Operational issues will result in premature wear or failure to the handles, hinges, seals and locking systems.

Having the door installation corrected will significantly reduce the risk of ongoing problems and the door becoming insecure, jammed or locked shut.