Legal action that can be taken if builder delays giving possession

I have booked a house at karanjade I have paid whole amount. All obligations are completed, I even have possession letter in my hand. But recently an issue arose between the partners of the building and hence the developer is not giving keys of our appartment due to issue among them. The builder isn't receiving our calls as well. I know we can file a case for possession. But I wanted to know that can the builder hold our keys just because of partners issue (also we have possession letter and house is also registered on our name) Also under RERA Act how much interest can we claim on delayed possession. Can delayed possession be of 1 month or so. Thank you for your response and time in advance.