Everthing else being equal, it can be caused by de-magnetization of the motors, generally this is caused by shock or high spike currents. A way to test them is if you have the holding torque rating is to apply the exact DC rated voltage to the winding and measure the holding torque by Lever arm and spring balance method.
The other measurment is to see if your DC collapses too much under load.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.