My garden is all in containers because I am in a wheelchair,and find it easier now to tend containers. I have raspberries and also a Blackberry in a hanging basket. Two years ago I bought a containerised Rhubarb. For the last two years it has grown well but I have not harvested it, as advised. However, this year , so far, 5 shoots have appeared (three weeks ago), but having emerged have not grown more that two or three inches. I mulched it well over winter, and fed it with growmore in March. Since then I have fed it with Miracle grow.. The other containers are growing well. Any ideas why the Rhubarb is not growing?