If you look at most aikido techniques, they use the extensor muscles, not the flexor muscles (making the distinction between flexion/extension vs. contraction/relaxation).
Thank you for your comment, Mary. I understand. In my experience, though, it is pretty common for people to activate their shoulder muscles and muscles in the upper back to pull. Maybe that could be one of the OP's problems, that he is doing this instead of relaxing and using his extensors and the power of momentum?
Maybe I am using the word flexing incorrectly but as I write this I can feel it in my imagination because I did exactly that for so long when I first began training.