Weak Rhubarb

I have a Timperley early rhubarb, about 5 years old. It is has always grown very weakly. The one thing I may have done wrong on recieving it in the first place was putting it in a pot, and maybe not watering it enough. It always rises long after my established crowns, and does not grow to more than 2-4 small leaves. This has been the pattern for the past 4 years.
The attached photos show the plants by my greenhouse, south facing in front of a hedge
Any ideas on how I can promote some health to it?

 

Could you not fertilise some soil with some manure etc, dig it deep, then transplant it?