With reference to your "underfloor weights" or "hammers".
It pays to check your RRPM is correct first.
Inherent in the Arriel and Arrius 1 FCU is a drift in the RPM with fuel SG.
So as your mean fuel temperature shifts with the season so will your RRPM.
The NR Tach is not accurate enough to see the difference - use the MicroVib Tach function.
Unless you have changed things in your aircraft drastically since it was built the hammers do not shift - it is the RRPM that has moved.
Playing with the hammers is a last resort.
