But as you said, in application, if they resist your technique, a new situation will arise, one where you can use another technique. In application you will flow from one technique to the next, because you can't expect any one technique to always work.
I think this is the key and as most things in real life it's often a combination of factors.
But the key thing is that uke shouldn't be applying resistance to cockblock the technique that he knows what is going to happen exactly. At this stage, it's important just to get the flow and learn the technique and introduce bit by bit of resistance so that you know he's not just dancing/giving it to you. A bit of resistance is ok but if you have real problems moving his arm, he's over resisting specifically to your technique.
When practicing a specific technique, resistance should only be so that you can feel how the technique should work and how the application of force from uke affects his balance and the technique. Without any resistance, Aikido is just a dance. However with too much resistance, the technique is jammed unless u apply superior strength which is not Aikido.
Now to the question, how do you know the technique works if it doesn't work against resistance? You have to assume uke does not know what you're going to do which is what usually happens in a more 'real' situation. Randoori is a good place to practice a higher level of resistance from uke where you are free to change and use whatever techniques you have learnt to go with whatever resistance he gives you. You know...if uke pulls, go with him, if uke pushes let him pass through sort of thing.
Now I have met highly ranked uke, that since they have had years of Aikido training, in randoori with more resistance, switching techniques becomes more difficult as they know what you're planning to do but you'll be surprised how you can appear that you're doing one technique and then change it into a different one. But that's a separate problem for another day