He was moving way too fast and making them too afraid to concentrate. Most Jedi can't use the force with that much precision in a stressful situation.Only after they nerfed the Jedi. Was it ever explained why the Jedi couldn't just toss Grevious around, you know, like how Yoda threw Palpy across the room, or force grab his lightsabers, or crush his skull?
Jedi did manage to tag him a few times, but each time he got back up. Mace Windu had the advantage of being an experienced master and getting him when he was standing still.
The thing about Grievous is he's supposed to be a dangerous Jedi killer. There's a reason he has a trophy collection of lightsabers. The second Clone Wars series had him pose a threat through a combination of guile and his own decent fighting abilities, while the first just had him as a ludicrously badass and unorthodox fighter no Jedi could have been prepared for. Both of those work, but it's hard not to miss the amazing lightsaber breakdancing abilities of the original Grievous.