If you've been playing the piano for a while, you've probably had the experience of being on a plateau -- just not seeming to improve even though you're still practicing as much as you always did. (This is a phenomenon that other discliplines, such as athletics, deal with as well.) It can be very frustrating. You wonder if you are doing something wrong. You might even have thoughts of giving up.
First of all, know that this is how learning usually works. It is often not a steady climb, as if on a hill, but more like a staircase: you make a movement upwards, then stay at the same level. Then another jump up, and level off again. You can even learn to enjoy the plateaus, knowing that your brain is preparing for the next jump up.
However, if you have a lot of plateaus that last for a long time, you're probably practicing wrong. Just playing the pieces over and over again is the worst thing to do. You can't "white knuckle" your way through a plateau. Instead, you need to find some more creative ways of practicing that challenge your brain, and cause it to form new "wiring" (neural pathways). I've blogged about many of these, but not necessarily as regards plateaus. Here are some things you can do.
Transposing. Playing the whole piece, or at least parts of the piece, in another key is supremely challenging for most people. The challenge is why it works. The goal is to transpose as much as possible by ear, but use the score as necessary. When you use the score, you are mentally calculating the distance (interval) from the original key to the new one. But doing it by ear is where the real benefit happens because it "drives the music into your ear." By that I mean that you ear will really know the music.
Playing hands crossed. Play the right hand's part with the left hand and vice versa. What this does is temporarily destroy muscle memory so the ear has to take over. Like transposing, it strengthens the auditory image of the music, that is, a deep knowledge of how it sounds.
Play with one finger, alternating hands on each note. This will obviously only work for a single line melody. You could do this with the main melody, the bass line, or any individual voice. If you are having trouble memorizing even just the melody, this would be good to do. As above, you can't rely on muscle memory so you have to use the ear.
Playing with eyes closed. You should be doing this a lot of the time anyway, as soon as you know the piece or passage by memory. If you haven't been doing this and you find yourself on a plateau, definitely include this in your practicing.
Practice the piece in sections in reverse order (assuming it is a piece of some length). Practice the last section through, then go to the next-to-last section, practice it and then play them both through to the end. Continue with each previous section until you are playing the whole piece. I can't really explain why this works, other than it is just a fresh perspective, but it does seem to help.
Take a break. Put the piece aside for a few weeks and take up some new pieces. The brain will develop new neural pathways from the new pieces that will, hopefully, assist in everything you play, including the piece you set aside. As mentioned, trying to force your way through a plateau is probably not going to work, and it certainly won't be fun.