One if the locking mechanism's is catching in the centre. it has never done this before . the front door was made and installed by Anglian Windows 9 yrs ago.
I had this problem earlier this year. i called
www.loockanddoor.co.uk
i had a great guy come out and replace my lock. havent had any problems since.
emma
January 2013
Check the hinges to see if they are a "flag type ". If so the door can be adjusted by using an allen key. They will be either 5 or 6 mm keys.
Check also if the locking points are engaging correctly or are they "knocking" when closing the door. If the door has dropped and is out of alignment then this can be rectified by glazing properly. What is known as heel and toe method.
Paul
March 2011
Door needs adjusting( dropped a little) adjusters on hinges turn top one back to frame bottom out away from frame.