It looks like your coils do not have the same amount of wraps.
I count one coil with about 8 wraps and the other one appears to have 10 which means one coil will heat substantially faster than the other. The gunk buildup on the faster heating coil, while the other one stays fairly clean (cold) further strengthens my suspicion. The 10wrap coil may also have a hotspot.
Take out your cotton, clean your atty and dry-fire it to see if both coils heat evenly and fire at the same time. If they don't, and it does not easly fix by gently brushing over the coil or tweezing (not likely in this case), redo your build. Always make sure things heat up at the same time and check for hot and cold spots on the coils, else you will be cooking your juice on the hot coil while the other one does not do much.
Remember that power always follows the path of the least resistance, which is why it is important that your coils are identical twins as far as wraps and leg lengths are concerned.
Not cramming the whole deck with cotton might also give you better results, but that is unrelated to your problem and ymmv.