Need solution for RWH issues in an apartment in Malad, Mumbai, Maharashtra - Water table levels unchanged

I have a query about rain water harvesting in a housing society located in Malad West, Mumbai.

The society has four buildings each having 7 floors including ground floor. The roof area is 3500 sq ft. While the builder claims that he had installed RWH in the building about 8 years ago, the residents see no change in water table levels. They fear that the water from roof could be going to under ground drains.

What could be the problem? Could it be that due to hard rock in that region, no change in water table is taking place?

Would it be better if they stored the rain water in sumps rather than allowing it to percolate to recharge the groundwater levels? Is there a rule of thumb to be followed?