OVERCHARGE INTEREST

OVERCHARGE YOU ON INTEREST

I have 2 home loans with FNB… How can your interest charge amount go up from one mont to another without a increase in the prime rate???? Your interest should go lower every month as you pay off not becomes higher ( unless there is a prime rate change) So disappointed and been trying to sort this out for more than 2 weeks with no response!!!!! Thinking of moving to another bank if this is how FNB do business