If it's an external facing wall it could well be that it has previously been painted with an oil based paint as opposed to water based. This may have been because if it's slightly damp, the oil based paint will not allow the old damp patches to show through. It's a bit like if you have a water leak in your ceiling and it dries out, no matter how much you paint in with a water based paint, the damp mark will still show through. If you coat it with an oil based paint it will stop the stain.
I would suggest that either you try a tester with some eggshell (oil based) paint and see if that sticks. If it does then either paint that particular wall with eggshell, or if you need to match the colour of the rest of the room then you will need to do a load more prep work. First of all sand down the wall to give the next layer something for it to 'key' onto, then clean with watered down TSP (trisodium phosphate), then put on a layer or two of bonding primer and then finally you should be able to paint emulsion over the top. It will take a while though so you may want to just stick with an oil based paint and have it a slightly different colour and/or sheen.