Can you tell me where you have it?
But something that would cause it to be a little bit warmer than the air temp.? Like a raised bed, by the house, stone path or cement or brick?? Southern exposure????

You said shade, too dry??? They like it moist and like compost or such.........

I'm not totally sure but when mine starts to go dormant that's the first thing I notice. It's in a raised bed and in full sun but, I compost it!!! So, I can sneak it a little longer into the season....But because of the bed and it's sun, it goes night~night early for me too!!

Alot depends on the weather, it's been cool here in the North East and mine seems to be quite happy, but I cut flowers off of it which also encourages it to keep growing...

what are the other two doing and where are they located???
http://www.wiseacre-gardens.com/plants/perennial/bleedhrt.html
http://www.mrgrow.com/plant/may.htm