I’m guessing your only problem is you are trapping where there are few minnows or at least no small minnows.Try setting in holes above and below road culverts,or under small bridges with holes of water under them.As far as using cheap minnow traps from Walmart,I’ve caught copious amounts of minnows from those them right off the shelf.
In a nutshell,dry dog food is about the best bait to use,bread balls work,but will break up faster than dog food.find some new places to trap ( holes rather than running creeks) and you should do better.