As a former, stand-up, steer by your pegs dirt bike rider in the 1960s, I have more recently found it difficult to teach myself how to get a knee-down riding a big Blackbird on the roads -- until I read Keith Cole's TOTW1 and came up with this discovery: "forcing" a knee down while turning was causing my bike to steer into the apex of a corner. The problem being, my cornering speed was not enough to benefit from the weight shift caused by the forced knee-down!
For this reason, either I (or perhaps the bike in its own infinite wisdom!) was instinctively "turning-in" toward the apex of a corner in order to counter the weight shift resulting from the forced knee-down. As a result, I have had no other recourse than to either: (1) give-up on the forced knee-downs, or (2) counter steer away from the apex with quick and usually awkward shoves on the outside handle bar. Perhaps, had I had the confidence, increased throttle would have accomplished this same purpose.
My new approach to getting knee-downs is to no longer try! I shift my body weight to the inside as usual, but then let my inside knee dangle FREE, which means it no longer comes down UNTIL I reach a cornering speed that actually benefits from the weight shift! More generally, I've learned that I need to stay RELAXED in the saddle -- sort of like a heavy sack of potatoes with shoulders hunched (I'm tall) and arms only lightly on the bars, and (especially) my inside knee LOOSE and NOT reaching for the pavement. When I force the knee downs, it feels like I am trying to remain upright on the pegs of a stationary bike when, in fact, I now know that I need to move before taking my feet off the ground!