Some door latches are just not installed snugly, while some doors warp with the seasons and are too tight part of the time and too loose the rest of the time. Typically, the strike plate needs to be moved over a fraction of an inch for a good fit. The best way to do that is to fill the existing screw holes with tooth picks and then put the screws back in just alongside the tooth picks. That will shift the screw slightly into the door frame in the direction you want to move the strike plate. Click here for more working details on moving the strike-plate.
But wait! I discovered a wonderful gadget that does this automatically. As you can see in the graphic, it has a stair step latch stop on the inside. The further you close the door, the further the door latch will step down the steps, keeping it snug under all conditions. It is the easiest striker plate ever to install, because you don't have to get it perfect the first time, and then it stays perfect even with changing weather conditions. You can see how it works by clicking on the animation above.
It is made in the US and has come and gone on the market, but one of our readers, Gilles Plourde from Burlington found it again in 2010. NRGides.com Thanks Gilles. Its cost of $4US each is why it could not compete on the retail shelves against what people thought was the same thing for 49 cents -- and shipping from the states costs more than the device itself, but this is a product that is just too good to let it get lost. Buy several and sell them to your neighbours and friends for $5 and you will end up getting yours free.