Do you mean it swings open or that it doesn't latch. For the former, try tightening the hinges. For the latter, in addition to the hinges, you might try moving the strike plate slightly to catch the door.
Sandra, we need a bit more to answer that question. As Steve said there are several things that can be done, but we need to know a bit more.
When the door is closed, or when you hold it closed. Does the top of the door match up even with the top of the opening? Does it appear to drop down over the side of the latch, or open up?
Stand on the outside of the door and slowly close it. Does the door hit the trim somewhere and not somewhere else.
Or do you simply have to push real hard on the door in order for it to latch?
Thank you for your help!
The door hinges seem tight enough, but the top line of the door is not perfectly level. The latch is not matching the strike plate area. I have another door with same problem which will latch if I pull upwards on the door knob and push it closed. The first door seems to drop a tad too low. Do I just move the strike plate and chisel out a bit at the bottom? Seems my home is "settling" a bit causing this to happen.
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resetting the latch striker plate is the short fix
Sandra...one trick I do a lot with "sagging" doors is to install longer screws in the hinges on the jam. this "lifts" the strike side of the door up a bit and often allows the latch to engage.
If its just a tiny amount you need to move the door up to aline with the latch, then you can do what Nichter suggested to reset the striker plate. But there is even an easier way.
Take some old lipstick and put it on the end of the latch bolt coming out of the door. Put enough on it so when the latch hits the strike plate it will transfer to it. Open the door latch so it does not rub on the strike plate close the door the release the door handle so the bolt comes out and rests against the striker plate to leave a mark. Then open the door to see just how much is needed to get the latch to properly close.
Using a Dremel tool, or even a small half round metal file, file away the strike plate enough to allow the door handle bolt to close as it should.
Moving the plate is an option, but you need to cut into the door frame a bit and reset the plate. While this is fine, if the door gets worse over time you will eventually need to pull the door frame out of the wall and reset it or jack that area of the house up and that will require you moving the plate once again. I use the Dremel tool, the smaller hand grinder with a metal grinding tool attached. It makes fast work of the latch plate.
If indeed the hinges are the cause of the sag then do as KMS said to use longer screws on the top hinge and tighten them up a bit more to pull the door up again.