Projectiles aren't great against Fox, or really any char with good mobility. They can just run around them or reliably approach with running shield..
Don't try random grabs against Fox. Wait for your setups, which include jab jab, Bair, fair, Nair, and anything that forces a knockdown (dsmash being a good one). If Fox full hops against you, run away-pivot grab is pretty free (depending on how they use double jumps)
Out of grab, you really have to get more than dthrow upB... Dthrow tech chase at low % guarantees a regrab or dash attack, if you're quick. Mid %s, get those utilt/dash attack staples out of dthrow or uthrow (%dependent). Above like 70,dthrow upB puts fox off the stage, which should lead to a Nair gimp. Any time after that, grab = upB or dair out of either dthrow or uthrow (just learn those combos by percent).
Any time Fox ends up on a platform above you, usmash under the platform is a surprisingly cover-all attack, and is unpunishable.