Ah yes, the old plateau problem. We've all been there I think. I'm more and more inclined to think that that's exactly the time when you can begin to make real progress, if you go about it right.
You need to be very analytical to decide to do next. First of all, is it a technical or an interpretive plateau?
If it's technical, where exactly are the hard parts? Which measure, which notes, which hand? Concentrate your practicing there for the time being. Try them slow, fast, hands separate, hands together, looking at the keys, looking at the music, eyes closed, eyes open, whatever works. Don't worry if it seems like you're spending a long time on even a short passage.
If it's interpretive, try changing things around. Don't always play it the same way. Bring out an inner voice, exaggerate the rubato, sing along while you're playing, make up a story to go with the piece (seriously!). Listen to recordings, but don't copy them slavishly.
In fact exaggeration is a good rule when you're learning a piece. Most amateur pianists play without enough contrast. There's not enough difference between their p's and their f's (or they just play everything forte throughout). The accented notes of a melody aren't accented enough, the staccatos aren't short enough, the dotted notes aren't dotted enough, etc.
My teachers always told me "when it sounds exaggerated to you, it's probably just about right for the audience." And of course, you can always bring it back down again if you go too far.
In other words, if you're bored with a piece, don't just practice it the same way every time. Work on different aspects of it on different days.
On the other hand, there's always the possibility that the piece really is too difficult for you -- or you've been doing it so long that you just can't stand playing it any more. In that case, then yes, move on to something else...