Dying blueberry leaves - what is wrong please?

The leaves on the blueberry bush that I've had for a few years are going brown, have spots on them and are dying. Its in ericacous compost which I top up every year, and the pot has a large hole in the bottom and is propped up off the ground. I have another blueberry bush a couple of meters away that is perfectly happy (included in the photos). What do you think is wrong with the sad bush please?