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.
All muscle movement is caused by one or more muscles contracting -- "activating", as you put it. The question is, what muscles? When a muscle isn't contracted, it's relaxed, which is why "Relax!" is so useless as instruction -- if we really relaxed, we wouldn't move at all!
Then you have flexion and extension. Flexion means to decrease the angle of a joint; extension means to increase it. This is most visible in the extremities, where we speak of opposing muscle pairs such as the biceps and triceps. When the biceps is contracted, that causes the elbow joint to flex; when the triceps is contracted, the elbow joint is extended.
My simple (probably simplistic) take on it is that many (most? nearly all?) aikido waza are based on extension, and that takes a little getting used to. Maybe our day-to-day actions use flexion more? I don't know. But I think that many people, when they think to themselves "make an effort" "move that person" or whatever, their muscle memory tends to fall back on flexion. So, obviously, it's not simply a matter of relaxing ("Relax!") the biceps; you need to contract the triceps, that whatsisname funny muscle on the underside of the forearm, etc. "Extend" is another command you hear a lot, and many people seem to think it's something ethereal (probably because so many people say "extend your ki", whatever that means). But it's not ethereal at all. It's simple body mechanics.