Please first compare your pension slip for the month in which you received a lower payment with the pension slip for the previous month and identify the differences (in gross pension amount or in your pension withholdings).
During the year, the differences you notice usually come from changes to the rate of withholding tax [PAS], which are imposed on us by the DGFIP.
The PAS rates notified to us by the tax authority every month are taken into account in the following month’s pension.
Your pension slips and the PAS rates notified for you are available in your personal account.