Well, as an electrical dude, my comment would be that the motor may have a permanent magnet for a field. After a few years, months whatever, if the magnet gets weak, the starter power diminishes. I would take an ohmeter reading of the starter to see what the reading is and compare to a good one although it will be very low resistance anyway. Next I would clean the commutator and check that a brush isn't hung up or worn out. Lastly I may be inclined to megger it at a lower voltage (300) to check for grounds or a shorted winding.
Or maybe these starters are throw away and you can't maintain them at all.