your frame has shifted and the locking throw is not protruding far enough into the hole on the jamb side.
Open the door and look at that spring activated throw that protrudes from the lock set. Squirt some wd40 into that area and make sure it springs back and forth ok.
Now see the hole on the jamb side of the frame that this protruding spring throw goes into? Several things have happened, one, the throw is not going into the hole squarely, the hole is too far from the throw, or There is debris in the hole that should not be there.
what to do, get some of your or your wifes or sig others lipstick, smear allot on the throw, turn the door handle to retract the throw, now close the door and now release the handle. the throw will leave its mark where it is striking the hole plate. Now, turn the handle to retract the throw and open the door.
Now you will see what the throw is doing. If its off center, then remove that metal hole covering and re adjust that plate either up, down, or over. Rescrew with new and longer screws or cram several tooth picks into the old hole and re screw again.
Now if the throw is not penetrating the hole far enough, then you need to add washers to the backside of the covering to protrude it out further for a good latch.
Wow, I do hope that you understand all of this, if not please come back and I will further address this simple fix.