The smaller fish seem to bump harder than the big ones most of the time. They are used to eating hard objects but I think a 4inch bait that weighs 4oz and is hard as a rock may seem suspicious and the larger fish are more cautious. My uncle is a fish ya know.
When I've fished them, I'm drifting in heavy current, and working the bottom. Rod goes up and down in fits and starts. After a strike, a little extra pull up and bingo. They seem to hook themselves through engulfing, too.