Delay in possession after registration

In your agreement it shall be clearly mentioned about handing over the flat and along with your payment schdule.Penal clause shall also be mentioned case of default .

If you have made all the pending payments, then the builder has to handover possession immediately aftter the registration is done, provided the construction is completed in all respect.

You can issue a legal notice to the builder demanding handing over possession if he is unnecessarily delaying the process. 

conditions regarding possession  is always mentioned in  agreement and builder is abide by that condition so you can take legal steps as per terms of agreement .