hi, if i where you i do the following first
You need to have a look at the lock keeps and also
have a look at cut out around door barrel behind the handle to see if there is a problem .
check to see if the door and frame is square and adjust if possible via hinges .
adjust dead lock spindles and lock strikers if needed.
basically you need to mark every thing up looking from the outside with a pencil with the door in the locked position and close it towards the frame to see if there is any thing not cut out or not adjusted properly....
it took me a few hours to do mind , i right pain in the dark with a torch ............so dont do it in the dark . i has no choice as work shifts.
hope that helps
jvc600
February 2010