Just talking to a buddy about that tonite as my two have same problem. he suggested that I check to see: if the rubber weather strip on the bottom of the door is hanging, and also see if the door itself is sagging on the hinges. before you close it, get it close to the strike and see if you can lift it into the opening or if it's already aligned. The fix he says, is to loosen the top two bolts and lift the door and retighten the bolts on hinge.
I've not tried it. so let us know.