Water dripping from my roof , seapage from walls in many area

Hi , Greetings!!! for the day . I vikash from Kolkata , we prchased a flat in Ground Floor in 2016 , In Early stage my walls has Seapage in some area which we complained and it was repaired by the Promoter as there is a Loandlor and Promoter Share. I have Purchased form Landlord . Now in my upper floor some one Shifted and aftere few months roof started seapage and now it is dripping. The earlier walls which was repaired also started seapage again and this is in all the Flats iin this building . One day i have asked the builder to repair the same but his answer was that he sold the flat and now he cannot as i cannot repair it lifelong. Its we are paying hardearned money fro repayment of loan and its feel unsecured . We have asked our other flat owners to write a letter and we will post it to him along with Landlord . Meanwhile i will also send a individual letter to him for repair . i need help how to get it repaired the damages done , we want to move to consumer forum for the same. So please Guide us . It will be great help . thanks vikash