Large Euphorbias died - why

I have 2 of the larger euphorbias planted either side of some steps.  They have been there for a year and a half, one did really well but the other, after a good start, started to die and now the other one has gone the same way - any idea what it could be?  Are they quite difficult plants to keep alive?  So fed up because I love them and they would have looked great there.
Thanks, Juliet

No, not cleaned the steps with anything at all.  And the first one died before the cold winter.  I might have honey fungus in my garden as apparently it is quite common in my road.  I have no seen any toadstools though.  I have looked up diseases and can't seem to find any that fits.   All the leaves turn brown from the bottom and slowly works up the plant and now just a bunch of stalks with brown leaves

Dear Yorkie, yes I think you are right, thank you so much x  It is very damp there.  I will just buy another and put it somewhere else - not so damp!