Usually the carbon motor brushes are getting worn. Alot of times there will be an accumulation of "carbon dust" from the motor brushes wearing down. This dust needs to be vacuumed out.
With the machine unplugged, unscrew the black plastic "brush cap" and carefully withdraw the carbon brush. The carbon may be worn down too close to the spring & wire portion. If that's the case the brush should be replaced. There are TWO brushes (one on each side of the motor) and if one is worn, it is advisable to replace both of them. Now with the brush cap off and the brush out of the hole, stick your vacuum cleaner nozzle over the hole and suck out the carbon dust.
If the brush length was ok and did not look worn, you can reassemble everything and try it out.