you need to adjust you door keep,its the middle one on the frame,the part of the keep that your latch goes into(usualy the top part)it should have 2 screws to adjust it,undo screws move it out a couple of mm,tighten back up and try your door,if still no good do a couple of mm until ok.hope this helps
shaun www.sdwupvcrepair.co.uk, December 2008