Back door suddenly won't close
This type of shifting/sticking isn't uncommon with doors. Sometimes it's settling, other times it might be temperature or moisture causing swelling.
First, check the hinges and make sure that the screws are in tight. If you didn't check the door jamb on all four sides, do that.
If the door isn't closing against the stop in the door frame, check to see where it's hanging up. It could be weatherstripping or the door/frame may need sanding or planing in one or two spots.
If the door is meeting the stop on the frame evenly and the door still won't lock you may need to adjust the strike plate. You need to determine if the bolt is hitting high, low or off to the side. If you can't tell put a bit of lipstick or other colorant on the end of the bolt, shut the door and engage the bolt. You'll see a mark on the strike plate where it hit. If it's only minor interference you can use a metal file or Dremel tool to shave off enough metal from the plate to get the lock working. In most cases that will solve the problem. Otherwise you may have to shift the strike plate.