Could it be that General Grievous's suit would have diminished any abilities to use the 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."

[From: Darth Vader's Armor](http://starwars.wikia.com/wiki/Darth_Vader%27s_armor)