Dunno where you are..but I also have mostly campanula failures.

My suspects for me are heat and overabundance of wet clay soil.

dbarron - I have clay and it's been hot and humid (z5NJ), so there's that. a2zmom--thanks for responding. After two different campanulas in 3 locations, I think they willl all go on my avoid list, though, from now on.

I don't know if location is much of a factor but the trailing or creeping bellflowers - two species native to eastern Europe and with unpronounceable species names (and Blue Waterfall is one cultivar) - are almost indestructible here. Tolerant of very poor soil conditions but wanting very fast drainage, They will happily creep and/or seed themselves into rocks and walls and other nooks and crannies. They looks terrible after flowering and are best cut back hard to refresh the foliage and encourage a later flush of flowers.

I can't imagine either favors high humidity, though.

Blue Waterfall is a cultivar of Campanula poscharskyana. It grows rampantly in our damp, temperate climate and is happiest with good drainage but moisture at the roots. It will not be happy in heat. Personally, I prefer C portenschlagiana which is more compact and less straggly. Both species self sow with abandon in walls in my area.

Just to compare habits ...

C poscharskyana

C portenschlagiana

It is rare to see either species growing in a bed or on flat ground. These will be self sowers. You can see the situations they have chosen.