Seems to me that peripheral neuropathy, meaning malfunction of the peripheral nerves, has a number of underlying causes. In some cases the cause is fairly easily identifiable eg diabetes, in other cases it is very difficult to find, so the medics call it idiopathic or cryptogenic. In some cases it progresses, in others it is stable or slowly regresses depending on the cause.

It doesn't seem surprising that treating the condition is difficult and that the success of different medications varies a lot from individual to individual. But this doesn't condone the lack of research, quite the opposite.