One of the most common conversations I've had with beginners over the years goes something like this: "Coach, I am able to do the techniques during uchikomi and nagekomi but during randori they don't work. How do I get them to work?"

There's a short answer and a long answer. The short answer is: Don't skip class.

The long answer is: The difference between uchikomi/nagekomi and randori is that in the former, uke is not resisting. In the latter, not only is uke resisting with all his might, he's moving around, breaking your grips, looking out for counter opportunities and of course, trying to throw you as well.

BIG difference!

So, how do you overcome that? You do it by coming to class and spending a lot of time on the mat. Sprinters spend a lot of time on the track, swimmers spend a lot of time in the pool, tennis players spend a lot of time on the court. This is how they get good. They spend a lot of time playing their sport. If you want to get good at judo, you've got to spend a lot of time doing judo.

Yes, there's uchikomi and nagekomi but that's just the tip of the iceberg. Uchikomi and nagekomi are to familiarize yourself with the mechanics of the technique. That's all. To learn how to do it against a resisting opponent, you need to do lots of randori.

Through trial and error you'll learn how to deal with the way your opponent grips, the way he moves, the way he blocks and the way he counters your attacks. And each opponent is different. So, you've got to try your technique against different partners.  

In the process, you'll be countered and thrown many times. Don't be discouraged. Don't let it demoralize you. Know that everybody goes through this. So, leave your ego at the door and try your techniques over and over again in randori.  

Over time — after a lot of trial and error — you will get used to the different types of reactions from your partners, and eventually you will learn how to overcome them.

Perseverance is necessary. Even if you are very athletic and naturally talented, it will still take a long time to overcome an experienced player who's had years (or decades) of experience. That's just how it is. There is no short-cut.  In short, don't skip class.