How to Improve
I've always been fascinated by the fact that some people, no matter how much they seem to be practicing something, fail to improve their skills.
I first really noticed this while playing fighting games. Games are a good place to look at learning patterns since you can have many repetitions in a fairly short period of time. This means you can easily watch the growth of successful people in comparison to the ones that plateau and then stagnate.
The biggest difference that I noticed between the successful crowd and everyone else was their mentality. The ones that continued to lose and got stuck in a rut usually got frustrated easily, blamed other people for being 'cheap', and complained that the game itself was unbalanced or broken. By comparison the successful people usually recognized the mistakes they made, took responsibility for when they lost, and whenever there was something that they didn't know how to deal with, they figured out ways to adapt.
We have a name in the fighting game community for the first kind of people who merely blame the game and don't take responsibility for their actions, expecting to use the same tactics repeatedly to win against every opponent. We call them 'scrubs'. A scrub can actually be a skilled player as well, however at some point they stop honing their skill, usually far, far before they've reached the upper levels of play. Somewhere along the line they decided that they just want to win, now they're good enough that they should just be able to defeat anyone they come across without much effort and so they stop working.
I've seen this scrub attitude in martial practices as well, where someone decides that there's nothing new to learn and they stop working to improve. When this happens it's your job to break out of this, or if you are a coach/instructor to break your student out of this mindset.
The keys to working through this problem are first to realize what is giving you trouble. This is best done with a video replay, be it in tennis or street fighter. Watch the plays that you fail at, and break down what happened and where you made a mistake, what kind of mistake it was, and come up with alternative actions to address the situation in the future.
Then go and practice it. You have to drill the areas that you are bad at the most until you get comfortable with them, practicing your predetermined actions to see what works and what doesn't. Eventually you should be able to come up with spontaneous creative solutions in the moment that address the situation perfectly.
It is common that one of your basic skills is lacking if you are having difficulty in a more complex situation. If this is the case you need to go back and work on your fundamentals. This can be anything that you fail to execute competently, in the correct moment, at the correct distance. You have to go back and drill these skills in a more simplistic environment until you can do them at their basic level, then begin to add more variables until you consistently use that skill in an isolated manner. Then go back to the more difficult situation and learn to apply that skill as a part of the greater game. Often this can be very difficult, simply having the ability to execute something in one environment, say a spike in volleyball practice, doesn't mean you can execute that same spike in the stress of a game as the opportunity spontaneously arises. This is why you have to practice applying the new skill to your game in a training scenario, since temporarily your overall ability will be decreased as you focus on integrating your new ability into your game as a whole.
This temporary dip is worth it however, since it usually indicates growth, and is necessary for you to break through plateaus.