I read somewhere that it's common for people to plateau at a certain skill level, but some people overcome that. My google fu is not working for me, but I think it was to do with changing the approach. I've also heard of changing how you view yourself, imagining you already have the ability, "what would X do?" (Usain Bolt actually peaked and burned out when was younger, regained with a new coach, less training, and exercises to compensate for a congenital spine defect). i.e. Also, while at a plateau at one specific thing, there are many other things where effort still works (not overgeneralize from that one thing). I'm also plateauing (at discrete mathematics), so I'm interested in the answer here. Many creative people talk about leaving something that they are stuck at, and doing other things (perhaps a short break, perhaps for a long time) - then the answer (sometimes) comes to them, when they see with a new point of view. Talking to people about it can help (even "explaining to a duck").
Of course, maybe there are attributes that we can't change, like height, that seem as insurmountable barrier. While these may be a limitation in themselves, we may be able to work around them (e.g. professional basketball players under 6 foot http://www.toptenz.net/top-10-shortest-nba-players.php). And, well, there are invasive ways to increase height, medically.
The cognitive phase where you think about what you are doing and develop strategies to complete the task.
The associative phase where you practice, eliminate mistakes and learn what works.
The autonomous phase where you don't have to think about what you are doing anymore.
It is likely you are reaching a plateau because you entered the autonomous cognitive phase. To get off this plateau you should : focus on staying in the cognitive phase by thinking about what you are doing. Set yourself goals in proficiency and most importantly, stay off you comfort by going (~10%)faster than your cruising speed or performing a task more difficult than you are comfortable with.
e.g : repeat the harder parts of songs you haven't mastered yet instead of playing the same old song you have mastered, type 10% faster on the keyboard and note the mistakes you make,...
Of course, maybe there are attributes that we can't change, like height, that seem as insurmountable barrier. While these may be a limitation in themselves, we may be able to work around them (e.g. professional basketball players under 6 foot http://www.toptenz.net/top-10-shortest-nba-players.php). And, well, there are invasive ways to increase height, medically.