brushed DC motor lost torque
 

Hi,
I am using small brushed DC motors for a project (unrelated to RC) and have observed a loss of torque (to the point of stalling in my mechanism) over time. Given that there is a lot of expertise in this community on small DC motors, I am looking for some thoughts.
I have measured the terminal-to-terminal resistance of the motors to be about 14 ohms on a good motor, 2-3 ohms on a dead motor, and 40-2000 ohms on a failing motor. I am most interested in the failing motor category. Upon dismantling a failing motor for inspection, I find low resistance between the terminal and the brush AND between the commutator pads. Therefore, my conclusion is that there is a poor connection (high resistance) between the brushes and commutator.
On failing motors, I am observing black residue on the commutator. After cleaning the commutator with isopropyl alcohol, some blackness remains (almost like a burn mark). I am unable to reassemble the motors for testing after cleaning since disassembly ruins the motor.
Does anyone have insight into why the resistance is increasing? Is there a way to avoid it? Is this a common occurrence for small DC motors?
Thanks!

I will add pictures. Actually, I don't have enough posts for that yet.

brushes wear first check those

Have you mounted capacitors?
protection - Why are capacitors added to motors (in parallel); what is their purpose? - Robotics Stack Exchange

Brushed motors do not last forever. Brushes/springs need to be replaced, comms trued back to new condition. If your motors are not made to be re-built, then you better look at replacing them when you notice loss of torque. Another option, go with brushless motors if your application allows.

Resistance will increase as the connection between the brush and the commutator degrades. This is normal wear and tear, the main downside to brushed motors. The dead motor having low resistance indicates there's conductive material between the commutator segments, creating a short, or maybe the epoxy on the armature wires has degraded, creating a short there.