It is so easy for uke to resist when they know exactly what you are going to do. Have you asked your instuctor about this situation?
This. At my dojo, it's better to take the opening that presents itself rather than attempting a technique even though uke's energy is completely wrong for it. (And at some point Sensei will come along and explain to uke why his attack does not allow the desired technique.) But different dojos have different training etiquette.