There may be few reason to happen, 1st, if th wall facing outsite, may be raining time only feeling wet. If the wall are facing toilet, then may be your toilet waterproofing system no good, If your house is condo, then may be leaking on the floor of toilet unit above will make your wall wet as well. and another posibility is leaking on your water piping.

Diff case have diff solution. advice you to ask contractor to see in the house, find out the problem, then u only can get a perfect solution on that.