loose ? tighten its preload adjustment, the loosened srem bolts in back stem lets that happen

with the bolt in the center cap tightened, increases the tightness , then you re tighten the stem around the fork bolts.

If its also stiff its not working right. take the fork apart and replace the whole bearing set,
or clean and grease it at least.

or bring the bike to a shop. your Cannondale Dealer, then they can see
what replacement parts you need to buy.