Let's take art - specifically drawing. Ask any adult to draw someone's face and you'll get something that resembles the work of a 12-year old. By that age we are well and truly in left brain logic mode. A face is round, as two round eyes, hair on the top and a weird triangle thing for a nose.
Instead of sitting back and drawing what we see we draw what we think it should be.
The other night in the dojo we were working on kakie (grasping hand).You'd think it's easy, just get hold of your partner's arm or wrist. But things are never what they seem to be. For example, the strength of the grab is in the little finger. The index finger doesn't clench, it stays relaxed to keep the muscles in the hand and forearm relaxed. Don't get your thumb in the way either = it's vulnerable to attack by a lock or twist or wrench.
You can "grab" without trying to grasp with the hand. Make a shuto then twist the wrist outwards and you get a curve, a hook, from your little finger to the wrist. This can be used to latch onto your partner's arm and snatch it down - once you've received the attack of course! Don't try to catch a punch in mid flight! You have to connect with the arm first and redirect or absorb the force spiralling it in.
Another mistake is to try to block then grab. A block will bump the arm away so you can't make the hold. You have to connect and .... whoosh..... as though you are gliding through.
Stance and posture work is yet another area of difficulty. To perform a technique correctly isn't the same as doing what you think is right. Are you having trouble with kicks? It's probably not the flexibility in your leg, but restrictions in your hips and posture (tense shoulders?).
Again, stances... side stance (mahamne neko ashi) is a problem area for many, usually because the rear knee collapses inward. The solution isn't "Get your knee over your foot" (how?!) but rather open the hips (push them forward gently tucking the base of your spine under) and sink. Keep your spine straight too. Relax your ankle. Is your body weight being supported by the leg muscles or knee joint?
Instructors have to be careful how they explain things. "Turn your foot more" doesn't help much - how can I do that? The chances are the problem is elsewhere, more than likely any problems you have won't be in the limbs, but where they connect to the rest of your body (hips and shoulders).
Get your centre correct and the limbs will do what has to be done to fit the stance or technique. Avoid doing what you think you should do, just let technique move you...