One of them has a very heavy follow. It takes a relatively large amount of force to communicate to her what I'm asking for, which makes her somewhat sluggish to respond to a lead.
My impression, as a lead that has been learning how to follow, is that it's nearly impossible to be a decent follow without being self-mobilizing. Good follows that feel very heavy are still self-mobilizing; they're still in charge of responding to the lead, using the floor and moving themselves from foot to foot. I would think that it would be extremely difficult to dance with someone who simply picked up their feet and put them down, and relied on the lead to physically move them across the floor. Maybe it's different in the really close ballroom holds?