Landlord/sublettor not returning my Security Deposit

I had taken a 1bhk aparment on rent from Grexter, a company in Bangalore, from Apr 1, 2017 on an 11 month agreement. This company acts as a sublettor for the houseowner. My rental agreement is with this company rather than the houseowner themselves. In Dec 2017, I informed Grexter about my intent to vacate the premises from Jan 31, 2018 by email, in accordance with my rental agreement, which specifies that I give them 1 month's notice before I vacate. I vacated the premises on Jan 31, 2018. The rental agreement states that my deposit will be returned (after subtraction of 75% of 1 month's rent) within 7-10 days after I vacate. I have provided Grexter my bank account details by email so that they can transfer the money to me asap. However, it has been over 20 days since I vacated the premises. I am yet to receive any money from Grexter. I have been trying to contact them repeatedly by email and phone. They have been deliberately avoiding replying to my mails or taking my calls. When I do get through, they say that they will "process the case tomorrow". That tomorrow never comes. I need advise on what my next step should be to recover my deposit.