Quite often, students need to prove to themselves or others that a technique is effective. In the early stages of practice, this should really be avoided. Just trust that it has a proven track record of success.
After a certain level of confidence is attained on performing a given technique, the student finds it's ineffective against a certain uke. He questions the effectiveness of the technique.
Most of the time, it's not whether or not it's an effective technique. It's the uke who has rendered it useless. Uke resisted it thereby changing the required technique. The new student doesn't realize that and tries to force the technique he is trying to learn.
Uke has an important job to do and that is to HELP people learn the technique that they are practicing. Uke should go along with it and not resist it. This is not to say that he should be a dead fish, but to be flexible.
A more experienced Aikido practicioner, when faced with resistance, will simply change to a different technique in order to utilize uke's energy and not go force against force.