It's known as early postwar AF steam "syndrome". Early in that the armature has flat commutators not the latter barrel-shaped ones. Assuming all other conditions* are proper, clean, and in good tune, then typically the only lasting "cures" are to 1) re-face the flat comms of the armature on a small lathe, 2) re-face the flat comms of the armature gently and methodically in a drill press using 100 -->1100 grit sand/emory paper (basically #1 w/o a lathe), or 3) replace the armature with one less worn. Other measures like doubling up brush springs, messing with combinations of fore and aft thrust washers beyond those that ought to be in there can make a difference, but not usually for long. At least that was my experience with dozens (possibly hundreds) of them over 20+ years of repairs.
*fore and aft armature bearings, axle holes/bearings in the chassis, drive gear teeth and centering, worm gear, brushes, brush springs, brush tubes, brush bracket, fore/aft thrust washer(s), quartering and condition of all 3-piece driver wheels, electrical connections from railheads to motor, etc.
