I think that if there is proper tension, then you will feel the joint be pulled (into the desired position).  if you cannot feel the joint being pulled even when you feel/see stretch in a given band, first try taking a deep breath, and let it out to relax the joint that’s being worked – let the band take the joint in whatever direction it’s pulling.  if that doesn’t work, then try more tension – one way is by simply moving farther out with the same band, the other is to use a stronger band.  certainly bigger people can use bigger bands – us little guys can’t use bigger bands because of our lower weight, relative to the pulling force generated by the stretched band. it is too hard to resist the pull of a stronger band and get into position.