I am concerned that there is too much resistance in the dojo. In particular, a stronger, higher ranked person should never block another student from doing technique. What is important is that we each get a chance to practice our techniques. Uke should give a strong honest attack, or sometimes a weak but still honest attack so that somebody that is new or not physically strong can still practice technique.
What do I mean by an honest attack? Often, I see uke not resisting, but not accepting the ukemi either. This does nothing useful for either party. It is not good for uke, because they are not practicing good ukemi – which is critical for kaeshi and henka waza, and also to avoid getting hurt. It is not good for nage, because they know that they are not doing technique, that uke is merely tanking for them. So an honest attack is one that simulates a full bore attack, while moving more slowly and using less force so that nage can successfully practice his technique.
I must admit that I like it when I get somebody that is difficult to do technique on. So I no doubt encourage the wrong sort of practice. However, you might have noticed that I am actually quite easy to throw.
So when do we get to practice aikido full strength, with full strength attacks? Any time that you are working with somebody of similar rank and strength, and it is clear that both parties have practiced enough to benefit from this sort of practice. If either party is finding it difficult to do the desired technique, then the other person needs to work harder at flowing and being a good uke. Any time somebody has to strain to make a technique work, they are no longer doing aikido.
There is a fine line here, because we do need to learn how to use our strength productively in a technique, but most people do this before thoroughly learning the technique and this is counter productive. Merely trying harder is hardly ever the most productive way to practice. Rather, we should be studying how our body and uke’s body interact so that we are effectively much stronger than our uke (angles and leverages rather than muscle).
Finally, you may be unable to do a particular technique on somebody, particularly if they know what you are trying to so. In the dojo, the correct response is usually to ask them to use a bit less strength. In the street, the correct response is to change technique. This is what Musashi calls “letting go four hands”, though “mountain sea change” is also relevant.