Most processing statements are written to be skimmed, not read. That is not an accident — the fees that cost you the most are rarely the ones printed largest.
Start with the effective rate: total fees divided by total volume. If that number is drifting up month over month while your mix of cards has not changed, something has been added.
The five line items worth challenging are monthly minimums, PCI non-compliance fees, batch fees, statement fees, and tiered downgrade buckets. None of them are laws of nature. All of them are negotiable.
Bring us a statement and we will mark it up with you, line by line. There is no charge for that, and no obligation at the end of it.