For several years, I have had some lilac dahlias which have just gone from strength to strength. Not sure of their name. A couple of years ago I bought a Bishop of Llandaff, as I so liked the deep red flower. Once planted, not far from the others, it deteriorated and died. Perhaps it was a sickly plant anyway, I thought. A few weeks ago, I purchased another 'Bishop' - it was healthy and remained so whilst in the pot. Having planted it out, it now appears to be deteriorating too. Is there any special care I should have given them first?