If the gap is even down the locking side, you might just need to adjust the hinges slightly to make the gap bigger. If the gap is bigger at either the top or bottom, you may need to adjust only the top or bottom hinge. If the gap is smaller in the middle, it is likely that your frame has been bowed inwards. If the door is catching at the bottom of the outer frame it needs to be lifted on the hinges.

Please also be aware that if the gap isn’t even all the way around the door slab to the outer frame, the frame may not have been fitted square and may need to be redone.