Fighters are almost always advancing their skill level. The more they fight and train, the better they get. At least, that's what we all want to have happen. At some point, most fighters experience the feeling they've reached a plateau, where their abilities aren't growing. I see it happen a lot with first and second year fighters and a good bit with some long time veterans. Realms with less high end, experienced fighters will see it happen more often as well.
For newer fighters, there are a few possible reasons they'll feel like they've reached a plateau. My friend covers one cause of stagnation on her blog. Another big possibility is that it is all in your head. As fighters advance in skill, so do the opponents that they will fight. Transitioning between fighting people new to the sport and facing people with a couple of years under there belt is a big jump. While you get better, the fights get harder, and that can lead to feeling like you haven't improved at all. You may have also advanced in skill enough that veterans are seeing you as a bigger threat, meaning they will fight harder against you.
To overcome that sort of mentality, try shuffling your place in the line every few fights. Sometimes finding different opponents helps give you a better sense of how you are doing. Off the field, try sparring the best fighters you can find. Their skills won't change as much, so improvements in your own fighting will be more obvious.
Actual stagnation in growth is harder to overcome. Once your primary style has reached a certain point, learning more takes concentrated effort to pick up new skills. This is when several fighters get discouraged, as it takes more and more effort to improve. That gratification of learning something new disappears, and it takes a bit is of fun out of the sport.
My first suggestion for people that feel stuck is to try a different fighting style. Picking up a support weapon or switching to florentine will build different skills than sword/board. You'll be less effective overall at first, as you need to learn new things, but that sense of improving and growing is refreshed. Not to mention, several skills translate from style to style, so it is a good practice to switch between them anyway.
One other suggestion is to have a goofy day. Yep, goofy day, you read that correctly. Plateaus make people have less fun at practice. Pick up silly equipment combinations, or start with no gear, have fun with it. Having a fun day of fighting will restore the drive to fight, and your drive to improve with it. Heck, you might even learn a thing or two.