Over the years, I've noticed that some colors just seem to be thinner than others, for whatever reason (I've got a bottle of Forest Green that's almost like water, it's so thin, and it came that way when I bought it). Shaking sometimes helps, as ẈĀŦƧĦĪƧ₣Ā₵Ė mentioned, but not always. As long as you're getting good coverage with it, you should be okay. Personally, I'd rather my paint was too thin than too thick.
I've also found that, if I use a particular bottle of paint for washes a lot over a long period of time, water can gradually accumulate inside the bottle from dipping the brush, which can also thin the paint out some. If you leave the bottle open for a while (say, a few hours, but not days), that'll give a chance for some of the water to evaporate which should restore some viscosity.