I've found that when a big flock has a sentry you either need to kill him or your setup must be perfect. We've had great luck with a piece of lightweight black material made into a flag about 18" long and 6-8" wide and attach it to the owl decoy at the talons. If theres any breeze the crows go nuts. I also put a lightweight black flag on one of those weighted decoy balls made for predator hunting. Another thing to try is one of those flea markey flapping birds or bats. (paint bthe birds black)
You can buy all of those cheaper than one mojo. I've had crows see that flag flapping under the owl and dive straight at it.I shot one 6 feet over the owls head.

One more thing. We have found that we do much better by keying in on small groups of crows. Oh, say 2-12. The bigger flocks always seem way tougher to fool. We drive around and spot them and then look for a place to set up on em.