Torque in relationship to Voltage
More voltage means more current which means more torque output from the motor. Different amperage implies you have a current limiter, and can adjust (limit) the drive current separate from the drive voltage. If that is the case the motor speed will reduce under the same load but with less current. Drop the current low enough and the motor will stall for a given load. Do not exceed the voltage or amp rating for the motor and it should be ok. If it stalls under full current (the maximum allowed for the motor) then the load is too much or the motor is too small. Make sure the load is well lubricated and free of rust.