Although I juggle solely for fun, I've often wondered if there were some ways of making the most of my available practise time. With this in mind, I contacted Professor Buckolz of the physical education department at the University of Western Ontario. It turns out that there's been a tremendous amount of research in this area, much of which is highly applicable to juggling. Almost everyone who has tried to learn a difficult trick has encountered a performance plateau. During this time it seems that your skill level is not improving at all, or even that you're getting worse.
Academics have some good news for jugglers here.
As you will see, it's important to make the distinction between learning and performance. Experiments have shown that while an individual's performance may level out or even decrease over the short term, the long-term rate of improvement is unaffected. Researchers have also looked at the effect of fatigue on the learning of motor skills. Near the end of a long practise session, you're having a rough time with difficult tricks and dropping a lot because you're tired. Personally, I've worried that this might reinforce poor juggling technique.