Do you have a door that won't latch? Are you ready to rip it from its hinges? Relax, it's probably a quick and easy fix. Although a number of things can cause your door to stop latching properly, in most cases the solution doesn't require major surgery.
In older homes, a door that won't latch is often the result of a latch that sticks. With the door open, turn the knob and observe what the latch is doing. When it is out as far as it will apparently go, grasp it firmly and pull. If it comes out further, it needs to be lubricated. If you don't have any machine oil or spray lubricant, or if you simply don't like the way they smell, you can use cooking spray or cooking oil to do this. Once the latch is lubricated, push and pull it through its range of motion, reapplying lubricant as needed, until it moves freely and easily.
If the latch moves freely but your door still won't latch, you need to determine how and where the latch is missing the strike plate. A good first step is to apply a small amount of lipstick to the end of the latch and close the door. You will be able to see from the lipstick left behind on the strike plate if the latch is hitting too high or too low. If it's hitting properly, make sure the opening in the wood behind the strike plate is as large as the hole in the strike plate. If it's not, use a drill or rotary tool to widen the hole in the wood so that the latch fits.
If the door is hitting too low, the problem is likely that the door is sagging. Check the door hinges, particularly the upper ones. If the screws are loose, tighten them and see if this fixes the problem. If the screw holes are stripped, remove one screw at a time, fill the hole with wood slivers or pieces of wooden toothpicks, and replace the screws.
If the door is still not latching properly, you can adjust the strike plate. If the latch is hitting the center of the plate but not making all the way to the opening in the strike plate, you can move the strike plate slightly forward without it being noticeable. Remove the strike plate, fill in the screw holes with toothpicks or wood slivers, and reattach the strike plate about an eight of an inch forward using self-drilling wood screws (you can pre-drill if you don't have self-drilling screws, but you need to use a wood filler to fill the old holes and let it set first).
If the latch is hitting the strike plate off center, you really don't want to move the strike plate up or down. The strike plate sits flush in the frame, and if you move it, you have a gap above or below it where it used to sit, so the adjustment will be noticeable, and unattractive, when the door is open. Instead, use a rotary tool with a grinding attachment to grind away enough at the top or bottom of the strike plate to allow the latch to slide home. If you don't have a rotary tool, remove the strike plate and use a file instead. Repeat the lipstick test first, so you know how much you need to file off.
Despite the numerous things that can prevent your door from latching, it actually only takes a few minutes to diagnose and repair the problem once you know what to look for. The aggravation it saves makes it more than worth the effort.