I certainly won't give you head... sorry, I don't swing that way. But, head retention is an entirely different issue. What is the recipes you are using? No, I don't expect you to type them all out, but head retention will be a factor of the grains, temperature, water profile, and glassware.... yes, glassware. If you use jet dry or any product with a surfactant on your glassware, that will certainly kill head retention. Carbonation is different from head retention in that it is a measurement of how much C02 is dissolved in a particular beer. You could have a beer with 4 volumes of CO2 yet still not nearly the head retention of another beer that say only has 2.2 volumes of C02. So to sum, without knowing all of the details of your process, we are really just guessing... but at least you have a place to start looking to control variables. Also, carbonation will likely increase a bit over time... some large beers will take several months to fully carbonate... 3 weeks is really just a minimum amount of time. Also, if you store your carbonating bottles in a cool room, it will take even longer... I shoot for about 68-72F for bottle conditioning temps.