the article I posted wrote:
A flat place in a learning curve, indicating a period of little or no progress (Fig. 33).The occurrence and persistence of a plateau depend on the nature of the task, the motivation, the approach of the learner, and the amount of previous experience he has had.
As it points out here, mindset is quite important. By allowing yourself to be frustrated or bothered in some way by the skill plateau, you are likely making it harder for yourself to actually overcome it.
the article I posted wrote:
There are several reasons for plateaus. First, the skill being acquired may have several distinct stages, each of which must be completed before the next begins. This is particularly true where a “hierarchy of habits” must be mastered, as in studying a musical instrument. The fundamentals of piano- playing are easily learned, but after that a discouragingly long period of little or no progress (the plateau) may be encountered. Once the student gets over this period, he enters a new stage of learning where hands and fingers are integrated and he has the satisfaction of playing actual pieces. A whole series of plateaus occurs in learning to type, since the student must acquire different habits for individual letters, words, phrases, and sentences. There is usually a period of no apparent improvement during the transition from one of these levels to another. Second, fatigue or distraction may temporarily slow up the learning process. The two often work together because tired students are easily distracted or lapse into daydreaming. Third, a previously learned skill may interfere with the one now being practiced; for example, reading teachers find that children who are used to reading aloud often find it hard to switch to silent.
Emphasis mine. I'm sure that is not an exhaustive list of why plateaus may occur. Osumania is fairly comparable to either learning an instrument or learning to type, so these examples are pretty good. The separate habits mentioned for words/letters/phrases is essentially the exact same thing as learning how to hit different kinds of patterns. But there's also the additional element of accuracy which isn't even covered by that comparison.
By far the most common problem people encounter is actually reading problems. Most of the time, when someone describes their problem with the game, it almost always comes in the form of "my fingers are too slow" or something to that effect. The reality is that in almost all cases, it has nothing to do with the physical speed that you can move your fingers at. It's almost always related to reading.
Reading a pattern is not something which either works or doesn't. Reading a pattern takes time. First, you must identify which parts of what you are seeing are important, then you must figure out exactly what the information you consider important is actually telling you, and then finally you need to translate that information into a series of motions that your body can carry out. All of this must happen before you even begin to move your finger. If this process takes just a BIT too long, you often have enough time to begin to move your fingers, but not enough to really properly coordinate that movement. It may also be that your brain struggles with a specific part of this process.
The article points out that other things may interfere with your learning, and it's quite possible that something external to the learning process itself is contributing to the plateau. As Delta386 mentioned, physical condition (and mental condition... how well you slept, etc.), specific technique or setup (wrist up vs wrist down, hand spacing, keyboard angle/distance from your chair and many other things) all may have an impact on things here.
Changing one thing, such as finger position, and expecting the plateau to just immediately go away is not a helpful way to look at things. Experiment with lots of things. If something helps, maybe stick with it. If it doesn't help, feel free to go back to the way things were before. But before you decide whether something is helping I suggest sticking with it for something like a week to make sure that you have actually adjusted to the change before trying to figure out whether it helped or not.