Stories of a short, fat bastard.

Range was on problem. The other is that its plutonium power source is nearing the end of its life. It wasn't "nuclear" powered. Instead if converted the natural radiation from a chunk of plutonium into electricity to power the craft. Once the plutonium has decayed past a certain point, the power levels drop until there is too little power to run the transmitters at a power level detectable at that range.

But it's mission isn't over yet. It will like as not float forever in space, bearing proof that we exist, or at least once did.