You should not be "paying ahead." You should be paying down principal. There is no benefit to paying ahead. No interest savings, nothing. If your lender/servicer does not let you make additional principal payments, start complaining very loudly. I had a mortgage company try to pull that on me when I added a large amount to the payment. They tried to credit future payments. I raised hell with the CSR who had no clue what I was talking about. I had to elevate the complaint to get someone that understood what I was saying and who grudgingly corrected their error.
If you contract does not allow you to prepay the principal, it's time to refinance with a lender that will. If it does, insist that you be properly credited.