Try another location. It may not like the lighting or the flow at its present spot. I had one that did the same thing. LiveAquaria recommends medium flow, lower to middle placement and moderate lighting.
Even then, you may just have to move them from place to place until you see them happy. Mine ended up about 1/3 way down tha tank almost under my circulation pump/
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....