During the course of strength training, I believe most everyone hits a plateau. That is, most people will reach a point where continued exercise doesn't seem to increase strength like it used to - if strength vs. time was plotted as a graph, it'd be a level region. It seems like I've become mired in a plateau of my own.
There are some various remedies for this situation. Most recommend varying the workouts - new exercises, new equipment, lower reps/higher weights or vice versa. Sometimes laying off from exercise helps, as does changing your diet. In the end, though, a friend told me from personal experience that the only surefire way is to keep plugging away at it, to work through it.
Of course, a little inspiration never hurt: