Faulty cellphone battery

I bought a blackberry cellphone battery (29/04/2014) at this shop and around August 2014t it started becoming faulty. I went there around September 2014 and they said they did not have any new battery (JM1) in stock . I went there several times and they kept telling me the same story. I have also phoned them ***) many times and still nothing has happened. Recently they even took my phone number but no one has phoned me to give me any progress on this matter. I even asked for a guy who is a manager there, I spoke to him and he even took my phone numbers but no one has called me back to explain what is going on. Please help me out with this problem.