I am reminded of many of my first day at different types of training, Aikido, jujitsu, judo, wrestling, even Gracie jujitsu (which was a lot of wrestling with shaking the opponent to break frozen muscles). The words of my first teacher come back, distraction or break his balance to apply your technique.
I see, many of you refer to going back to the basics, but aren't we really looking for the point of distraction to break the balance to use the technique?
The other thing to look at is ... are you hurrying to get to the end of the technique without taking the time to do what is needed to get there?
That is one of the treasures of my teacher in Aikido, half my size, less than half my weight, yet he still finds a way to take my balance, distract, before attempting to perform irimi, nikkyo, etc... You must commit yourself to always being the beginner, always relearning the ways of balance, distraction, technique. Other wise, you will frustrate your partner and yourself as you attempt to overcome force with force.
Sometimes, in line throws, or towards the middle of practice, I will give some resistence to make practice more than throwing a loose dummy all the time. Or ... as long as sensei doesn't disapprove, I will tell my partner they will now have to properly do the technique before I will go. Not like a buffalo, but once I feel the beginning twinge of pain that forces movement, or imbalance that forces movement, or energy that has finally been places at the point least resistence overcoming a relaxed posture.
Why? Because as you get better and better you will need to not only feel applied techniques but be able to increase your feeling of what can and can not be done withing the conditions present? Something like ... it is easier to roll a round rock than a square one? A bit simplistic, but I was seeking an energy coefficient.
Strong uke, tough uke? I guess they still have balance and no distraction.
Don't forget how the body is weak in the oblique angles, that help too. (but if the suckka don't play nice, a shot in the throat couldn't hurt?)
A little secret ... there are four pressure points on each joint of the human body. Watch your sensei when they grab wrist, elbow, or hand, they are causing one of these points to tell the body to respond with movement or pain. Pay attention to the way they are grabbed and moved. You can feel these points on yourself with very little pressure, but see your teacher for the exact way to use them. This will clear up a lot about how to move big stubborn people. Jujitsu is found in many of our Aikido techniques. Use it.