Posted: 19/04/2015 at 17:05
Tim, you clearly suspect roots are the issue. Yes, after flowering I,would dig it up with good rootball and check. I think you will find it will dig up easily with small rootball. Gently try to release some roots and then look at the planting hole. Enlarge it and add ericaceous soil, bark chips, compost. Replant and water it well.
Some may suggest digging up in the autumn but that's when flower buds are forming or have been formed. Do it now and it's got time to grow well and produce its flowers for next season