Possibilities are
1. little dust collected on helical springs of the movement.
2. the conical pivots develop deformity and / or collected dust.
3.dust accumulation between the magnet and the coil frame (could also be iron particles)
Solutions,
If you have stable hands and skilled to handle micro things under magemtic field,
you can take out
hold the metr needel and while holding , inspoect and slow blow off the dust
and a have aclean seethro the magenet for dust
.
f dust alone
blow it
if iron particles, perhaps you need a small antimagnetic wire, gor guiding the partciles out VERY PATIENTLY.
later fix back . it should restore
again , be careful, as unknowingly we are likely to damage more than attend and restore.