Al----In my experience, bitter smoking for cheese can be attributed to a couple things or a combination of things-----#1 in the smoker for to long for the type of wood, usually hickory, a strong smoker. #2 to heavy of smoke, a fast, hot burn can produce a lot of creosote that can coat your cheese, a fine thin stream of smoke often produces better results. Top and bottom vents in the box that are adjustable can really help, but has the down side of pulling in hot air if the ambient is high---I find that constantly moving smoke helps prevent unwanted bitter deposits on the cheese. Have had some of these same issues with meats, but they seem to be much more forgiving then the milder flavor of cheese.
Qdog