Could it be that General Grievous's suit would have diminished any abilities to use the force.Force? Vader's suit was made via Sith alchemy making it compatible with the midi-chlorians. Grievous's suit is almost entirely robotic apart from the brain, which was what gave the robot its advantage of sentient forethought and equilibrium.
Vader's suit:
"The suit was constructed using various methods of Sith alchemy which served to augment Vader's severely diminished physical strength and vitality."