I'm thinking that those fish were down close to the thermocline and above that there is oxygen and below it, that's very low. If I went too deep in conditions like that, minnows died very fast. My standard advice when see but can't catch is to downsize and slow down. You may have thrown every Bobby you had but did you change the head weight? The one you caught on a crank was probably a reaction bite, not dumb luck. Stay with em, try different things and let us know what works. You're teaching us too.