Most of the time I can't use aikido on a compliant uke.
It is more difficult to do aikido techniques on aikido students at keiko, even if they are somewhat compliant - because they know what to expect, so they can easily block it, even unintentionally.
When an aikido student really resists a technique, it can be very difficult to do it. That is no mystery, and it is the same for any martial art.
The ingredient of surprise is a necessary part of any technique, so a compliant uke sort of has to pretend to be surprised