PLL, I have the same problem. To me it's really about the length, or maybe the fit at the ankle: the pressure on my kneecaps happens when there isn't enough length in the tights to let my knees flex. If they're zipped, then leaving the bottom couple of inches unzipped (and wearing longer socks to compensate) lets them ride up toward my calves and put less pressure on my knees. If they're elastic cuffs and not too tight, you could try just setting them higher on your ankles.