Rhubard is normally planted in winter when it is dormant; it then starts growing in the spring. If you really mean that you have just planted it when it is in growth with stems and leaves then it is not too surprising that it has flopped. The root disturbance will have upset it and it has not had time to put out enough new roots to sustain the plant. It will look sad this year, die back at the end of summer but then should come along strongly next year. Don't cut off the stems or leaves until they have died off and gone soft. These put energy back into the root.

Thank you for your answer. That's news has made me really happy.

Also try not to pull any stems next year - it will need a full season to establish.