Zest Money - Seven months The zest money company does not repay interest

I purchased an iPhone ** gadget in December ****. According to them, they will charge interest at first, but after a few days, they will refund the interest to my bank account. Only they repaid three months' interest to my bank account. Following that, I received no interest payments from them. I spoke with zest money customer service several times, and they still did not return my interest amount. They kept telling me that I will get the money in ** days. I waited about two months and still did not receive that amount. Today, October *, I called customer support, and they told me that I will receive the money on October **. I have documentation proving how many months I received the money.