Plateaus in learning have 2 major causes: the first is a transition between different stages of study and depths of knowledge. Initial study for recognition level is quick and easy, memorizing for the second level is not - you seem to stop learning despite working harder. If you continue working, however, you will again begin to see progress. Then, when trying to be able to put together what you have memorized to explain it in your own words and your own way, you will again feel slowed to a crawl for a time. And yet again, when you start trying to do original work.
The second major cause of plateaus is more common - a simple waning of motivation. Sometimes, there is a good cause, like tiredness from working too hard on something. More often you tried working from enthusiasm alone or from someone else's prodding, and when your enthusiasm waned or the other person stopped prodding you, you had no firmer motivation to back it up.
A third cause of plateaus, for physical work and exercise, is overtraining. Overtraining can be a simple failure of motivation, but it can also result from not giving your body enough recovery time between significant bouts of intense exercise. When you have to do hard, physical labor every day, as I did when doing landscaping, you have to space out any physical training more than someone who is in a less physically demanding job. Fortunately, the periods of daily, hard labor were only a few weeks at a time, so I could work around them.