hi Lyn, the screws become loose because there is a problem with the door where it's sticking somewhere, although the sticking may have been rectified by weather conditions, such as in summer the timber dries out, making everything work, but in winter time the wood swells, which will make it difficult to close the door, putting pressure on the screws.
You can put a match into the hole so the screw will tighten up, that will work, but if the latch on the door won't connect with the hole in the door jamb, then you can do something else.
Have a look on the door jamb and see if there are any scratch marks on the door or on the door jamb and let me know if there are.
Put something on the door latch, like some butter, then try and close the door, see where the butter is on the door jamb and let me know.
Please let me know if you can't understand what I have said, sometimes it is difficult. Geoff.