Help with Poorly Rhubarb

I'd be inclined to dig it up and see what's down there. sounds like either some pest or perhaps a large rock in the way. At the same time check the lower area for moistness. If all seems well regardless, then I'd move it to the far side of the first two into a freshly dug hole with lots of old manure, handful of bonemeal, or blood, fish and bonemeal, and lots of water before planting. Problems aren't always obvious, but a new spot usually helps.