Purchase a under construction flat

Have you taken possession of the flat?  If not what is the present status of construction ?  Have you gone to the allocated flat or obtained EC or confirmation from a builder that the flat was not allocated to some other to defraud you?   Have you issued any legal notice demanding registration of the flat in your name?  Are there some more victims like you ?  Have you contacted the builder in person with a request for registration and what is his reply. Contact local advocates immediately and take his guidance.  If there are more victims, form as a group and proceed legally against builder as per advocate's guidance.

Have you been put in possession?

Move under the provisions of RERA, through a local lawyer.

You have very good case and shall get desired relief.