All conditions for growing seem to be covered and the tree is about 20ft high
I don't see your location listed, but these aren't well suited for a humid, moist area. They also don't tolerate phosphorus, so be careful if you fertilize, which they don't really need. Was your tree grown from seed? If so, it can take 10 years to bloom. Incorrect pruning can cost the tree blooms. Check out its other growing conditions listed in this article:
If those conditions have been met, it might be a good idea to check with a local nursery or extension service for more suggestions.
I have a Banksia that has grown to 15 feet and it is shading my neighbors solar panels. I want to prune it back to 8 feet but that will cut off a large amount of its vegetation. Will the remaining branches sprout new growth. Can I do this now in September in California
It depends on the variety. About half of the varieties will tolerate hard pruning, the ones with lignotubers. There is a list of those in the second link near the end of the file. Most also flower on "old wood" so cutting into that will delay flowering. The best time to prune is after the main flush of flowering in summer.