First off, you can't just blow into a call and expect results, you need to learn how to manipulate your particular call. Mouth calls are for when you can hear them, but they're not in sight. That lets them come to your area and see if they can find you and the food you're talking about...
Second, it had better be a really good bb gun for you to go after crows with it, they have a kind of plate in their chest, if I'm remembering right, and you have to shoot them in the head.
And no, the electronic call won't work better, because the crows have to be able to hear it, and they know that other crows don't call quietly (have you ever heard a crow calling softly?), so they wouldn't land for a shot.
Basically, they have to think that you're another crow that has found food, not one that wants to fight, because they'll fight in the air. They land when they eat, so that's your best shot.
Try this sequence with your hand call:
Start with "fuu", then "all, all, all" Make sure your tongue makes contact with the roof of your mouth on the 'all's, and stop the air on that point.
After doing that once or twice, about a minute apart, just do the "all" in sets of three and six, alternating, but don't overdo it. Stretch the sequences over about fifteen minutes to half an hour, and if there's any in the area, they should come running.
It works for me, maybe it'll work for you.