1357ex Pro Faint prints

Hoping someone can help. We have 2 1357ex pros and have been having recurring problems with the print becoming faint. Our service provider always fixes the problem but it never lasts long. Changing the developer seems to help, but again, doesn't last long. Based on other posts I've read we appear to be set at 750 for toner supply and 10 for patterns. Both machines have the same issue periodically. we mainly run A3 (or SRA3) and a mixture of plain & pre-printed stocks (most under 120gsm). Does anyone have suggestions for what could be causing these problems?

Thanks

Michael