It may have some disease or pest, and they can be impossible to see, some of them are so tiny.

I'd try mixing 2 TBS vinegar with 1 cup of luke warm water and washing the whole thing with that. Roots and all. Replant in fresh potting soil.

That should either wash off or kill most parasites and many bacteria that could be harming the plant. Then give it some fertilizer - that'll help build it back up.

You could buy Neem and treat it, if you have other plants. But that stuff is not cheap, so I would tell you to replace your fuscia instead of spending that much to treat just the one plant.

Neem is awesome though. It's made of the seeds from chinaberry trees, and it's mostly harmless to beneficial insects like bees. I spray late in th day when the bees are not around, and the spray has time to dry before they come back out.

One spray took care of leaf miners in my columbines. We even saved the filbert and sandcherry trees from total ruin due to Japanes beetles with two applications. And the spray smells like oranges.