The dwarfing, leaves browning off and lack of flower suggest there is a problem with the root system to me. Sounds more like an environmental problem than a disease though.
I would dig the plant up and dig the soil underneath it. If you come up with a big lump of clay, that could be it. The roots would have not been able to penetrate down through it. If that plant also gets the downpipe from the roof emptying next to it, waterlogging could be adding to the problem.
If you can improve the soil and add some drainage like grit, the plant should be fine to go back in. Or you could take some root cuttings and grow a fresh batch of plants