What type of waterproofing? If it's the type where they paint something on, I'm very doubtful of the effectiveness.
You really need a good specialist to look at it. If it was poor workmanship during slab casting, and the repair is not properly done, you will have a recurring problem.
Bathroom areas are always problematic because of the drop slab and the skills of the carpenter and concreting group. I would suggest that the location of the leak be isolated, hacked to expose good concrete, cleaned and re-sealed with non-shrink grout. Then apply your waterproofing coat as per specifications. Usually another layer of cement screed will then be applied before tile works.
Good luck!