Flat — Water Leakage problem in our flat

Hi, We own a flat at First Floor and our kitchen roof ceiling is getting wet and we can see the water stains. Also in our bathroom ceiling has the same water leakage/stain problem. Second floor owners are not allowint us to look the issue and to rectify that. Once we got the permission and applied some water proofing solution in their bathroom. That solution was exisits only for a few months and again we are seeing the issue very badly. Highly appreciatable for any advice.