Maintenance amount collection

We(Association) are currently in the process of taking a hand over from builder. There are approximately 548 apartments and roughly around 60 flats unsold. We had initially paid 30000 towards advance maintenance. Its been a year and this amount has now exhausted. We now need to collect amount from owners and the builder mentioned that for unsold apartments they will pay the maintenance to the Association, however will not pay for unregistered apartments. Unregistered apartments are those apartments , where the owner has been intimated to take possession or come forward for registration, however is delaying. The number of such flats are approximately 70-80. Now since these people will not pay or delay, the load of their maintenance will fall on other owners , which we feel is unfair. hence we want to check if its right legally to ask the builder to also pay for these apartments as they are still unregistered. This will help us in adding money to our maintenance kitty and will apparently decrease the load on other owners.