No matter how much care a cabinet installer puts into hanging cabinet doors and getting them straight, the odds are they won't stay that way forever. The problem usually isn't with the doors, unless the climate in the kitchen is moist enough to warp the wood, but with the hinges. If they're adjustable, the adjustment screws often loosen over time. If they aren't adjustable, normal use of the doors often loosens the screws holding the hinges. You can usually straighten the doors, even if they're warped, with a few basic tools and supplies.
Open one of the doors that is hanging askew and pull on the outside edge. If the hinge moves, look for an adjustment screw. Many cabinet hinges are connected by a movable bar, and the screw must be tight for the bar to be stable.
Place a torpedo level on the top of the door and move the door until the bubble is centred. Tighten the loose adjustment screw on either of the two hinges with a Phillips screwdriver. Close the door and verify that it stays level when closed.
Tighten any screws that have worked themselves loose from conventional hinges that don't have adjustment screws. Sometimes, tightening the screws will be sufficient to make the door hang straight. If the door is still crooked, or you can't tighten some of the screws, you may have to reset a hinge.
Unscrew the hinge from the cabinet and the door and fill the screw holes with epoxy wood putty. Let the putty set for about two hours, then screw the hinge back onto the door. Hold the door against the cabinet and level it with a torpedo level. The hinges should be flush against the side of the cabinet.
Mark the holes for the hinges on the cabinet with a pencil, then remove the door and drill holes on each mark with a 1/8-inch drill bit. Set the door back in position and drive a screw into each hole to hold the door securely. Open and close it a few times to make sure it's straight.
Straighten a warped door by making a makeshift truss. Measure the distance between opposite corners of the door and use a hacksaw to cut a piece of 1/4-inch threaded rod that is about 6 inches shorter than the distance you measure.
Drill a 3/8-inch hole in one side of a 1-inch metal corner bracket, and then repeat with a second corner bracket. Screw the brackets to the inside of the door on opposite corners along the warp so the holes are facing each other.
Cut the threaded rod in half and insert one end of each half into the holes in the corner brackets. Join the rods with a threaded coupling, then place a nut and washer on the ends of the threaded rods sticking out the corner brackets and tighten the nuts with a wrench.
Tighten the coupling with the wrench to increase the tension of the rods and draw the door straight.