I planted a Fine Wine Weigela 2 years ago. It gets plenty of sun. The plant is getting very sparse on the bottom, but I have a lot of new growth on top and it's even starting to rebloom. I haven't pruned it yet. If I cut it back in the fall would that help?

You are supposed to prune if Spring after they bloom so you do not cut away next years flowers.

But weigela can take a cut anytime and recover. It may take more than a season to fill out nicely from a hard cut. I haven't had a weigela yet that I didn't have to do this at some point.