I have a delta kitchen faucet with the ball cartridge valve that is about 12 years old. It started leaking out of the top recently. We shut off the water to replace the seats and springs and now there is reduced flow from the tap.
It is both hot and cold side. The shut off valves on the lines are working OK.
Is there something in the cartridge that could be blocking flow?

With a ball it is critical you line the slot up with the projection on the body of the faucet. Are you sure you did that? Otherwise, the holes don't line up.

Would this help? Good luck, Tom