Are you pruning out the old canes each year and mulching them with well rotted manure or compost?

I think raspberries prefer the soil to be on the acid side.
Mulching regularly with well rotted grass cuttings or pine needles as well as the manure might help acidify it a little.
Do it now so that the new growth will benefit. If the bed has been neglected you could dig out all the weak canes in the autumn and keep only the strong ones. This might entail digging and replanting, but you can use the canes you have already - they should respond well to feeding.

Also make sure that surrounding trees haven't shaded too much over the years. They are somewhat shade tolerant, but they need the equivalent of at least 4 hours of direct sun each day to bloom and bear.