Hi, i am a upvc manufacturer and fitter, make ure that you have the barrel the correct way round and that the black piece that rotates goes in between the backset of the lock(the black piece is not usually in the centre of the barrel so make sure you have it the right way round).
if that doesnt work does your door have but hinges? if so then nock out the pins and remove the door from the hinge side, this way you can see what the problem is.Also if it turns out the lock is broken there are plenty of places you can buy locks and spare parts. depending on the lock they cost around £30 to buy and you can find out the make and type on the lock itself. dont consult a locksmith speak to a fitter and if they cant help(which i doubt) then try a window company. there are several things it could be but without seeing it its hard to asess. Hope this helps if not contact me at alanjolliff@hotmail.co.uk
Alan Jolliff
May 2007