This is normal. All else constant, the lower strings go dead first, because there are larger cavities in the windings to catch dirt, sweat, dust, and skin.
The best remedy is a new set of strings. For the other 99.99% of us who can't afford new strings as often as necessary, the next best remedy is a soak for a day or two in denatured alcohol - get it at Home Depot in the paint section. Put some in a deep tupperware container, roll up your strings, and set them in it for a while. They come out sounding just like new. Really.
I usually keep two sets of strings around per bass, and rotate them on and off as needed this way.
Best luck!