My robot's motor has begun to spin slower, but it works perfectly if I use a different adapter with more voltage.. what does it mean?
In any case the ac/dc adapter need sufficient current to drive the robot. 1000ma seems to be sufficient for this application. Insufficient current will result in erratic behavior and or slow motor.

too low voltage could  result in slower maximum speed or even insufficient voltage for the logic/MCU

Too high voltage could result in overheating / thermal shutdown. ... or even permanent damage!!! Because of exceeding absolute maximum rating of components