Something I'm starting to realise is that there is necesaarily some cooperation from uke, as after the first throw (when you're training) they know you're going to do the same again - and therefore block it if they want to.
Also necessarily because it's the only way to learn the form
. Once you've learned the form you can study a more free form approach to practice that allows counters and resistence so that you can learn how and why the technique can work.
I think what happens sometimes is that this is unspoken and people forget to take on this practice as they progress to the point where they're discovering that the "techniques" don't work. A more advanced practice certainly.
It is very unfortunate that, to train in a formal way we have to go through repeated techniques of one type, whereas ideally we want to be changing our technique every time to respond to that particular uke and that attack.
You just described the difference between learning form (basics) and learning principles. We need to move onto the more advanced practice of adjusting to the attack and letting any technique happen as we become more advanced.
This happens in most dojo I've visited and may also occasionally be the source of the common complaint I've heard (and voiced) that people aren't doing their techniques the way the teacher demonstrated.
There's a time and place for each kind of training.