The gap between learning the mechanics of a technique to getting it to work in randori is a huge one.

When learning the mechanics, you usually start with uchikomi. This teaches you how to enter into the technique. Uchikomi is very useful for this. But it doesn't teach you how to complete the technique.

For that, you have to do nagekomi. Doing lots of throws on the crash pad is really important to get the feel of the throw. And it's important to do it on a crash pad because you want tori to throw with full force (uke won't last very long if they are thrown on tatami like that).

But Nagekomi alone won't get you there because in nagekomi, uke is not resisting. There is no grip fighting. There is no blocking. There is no attempt to counter. It's all cooperative. That's okay for learning how it feels like to complete the throw but it doesn't teach you how to throw in a full-resistance situation like randori or shiai.
This is why randori is so important. You have to try the throws in randori. Many players are reluctant to try new techniques in randori for fear that the technique might fail or might get countered. But you have to be willing to go through that in order to figure out a way to make the technique work.

Every time your technique fails or gets countered, you've learned one more way how not to do it. Learn from that. And don't repeat the mistake. Try again, with a different approach. If that fails, try again, with a different approach. Repeat this process until you find an approach that works. When it does, try to replicate it and see if it works a second and third and fourth time.

There's one more thing you can do that will help with your technical development. And that is visualization. Close your eyes and imagine doing the technique. Play it in your mind, repeatedly. Imagine doing it in a randori situation with your training partner or a shiai situation against your rival. Most of us don't have the luxury of top Japanese players who get to do randori for up to two hours a day. So, we have to do randori in our heads.