Patio door lock out of sync - wont unlock?

The system seems to have been out of sync for a while but normally after a bit of fiddeling it locks/unlocks. Unfortunately the key sheared of in the lock so I have taken the mechansim out to replace the lock. Now trying to put it back I can't get the mechanism to unlock. The key mechanism works fine but the system on the side that you pull up and down in addition to the key lock wont disengage. There is a sprung horizontal section that comes out to engage with the other door. It springs out but there doesn't seem to be anything that pulls it back. The only way that it seems to work is if you push the horizontal section back in - obviously not possible if the door is shut....help!