there were some errors in my bills. But no-one help to fix them.

hi, there were some errors in my bills. I was charged with a wrong service item, LMTD Basic HD. I contacted your agent several times. EACH TIME, they told me they will give me a credit to refund the wrong charge and cancel the fee from next month. Each time, they told me that this will be the last time that happened on my bill. But I didn't receive the credit/refund and I was still charged by the wrong bill item for several months. Please help to fix it. Thank you.