Heavy steering

Even with a power steering, the steering on my car becomes quite heavy, requiring me to pull and push the steering wheel. Moreover, the steering does not return to the centre automatically. I need to maneuver it.
I have changed the power steering pump but the problem persists. Could it be the problem with the steering racks?
I last changed a pair of reconditioned steering racks last year, but it didn't seem to solve the problem.