it is 'probably' the felt not seating (pressing) on the string snugly. One problem is that Oscar Schmidt and the like did not use the best felt, and if it is a used instrument, there will be little groves worn in the most used felt bars...gargoyle has a point in that, if you raise the compressed felt with a needle, you may be able to gain some time before the true solution, which is to re-felt the bar or entire instrument.

(with some looking it is possible to find scraps of sheet felt with different resiliancy and longer life)...I have NOT tried this in the Wash DC area yet, as I still have some)..[you can simply buy standard felt from the makers.]