Sounds as if your Rhubarb plants are old.
Every 5 years or so, you need to lift the crowns and divide them. You have to discard the old 'woody' middle bit, but keep the younger outer growth intact, and then replant the younger stuff.
If you do this work in Springtime you can give the plants their annual dressing of well rotted mature at the same time to give them a good start to the year.
Hope this works! Best Wishes, Mitch