[Cob] Plaster failure question

[Georgie Donais]

I plastered a cob wall last fall with lime. It was type S, so sometimes was 
soaked overnight, often less than that. I followed all the instructions (I 
think) in Cedar Rose's plaster book for mixing and applying, and added a 
small amount of pozzolanic additive to speed up the curing process a bit 
(Marlin brought me up to speed on that). I had trouble keeping the wall 
covered and moist for a week, with all the curious onlookers who kept 
pulling off the tarps to look.

The plaster hardened nicely, and seemed to be in good shape going into the 
cold season.

Over the winter and spring, it became obvious that significant portions of 
the plaster had pulled away from the wall and was primed to fall off, or be 
knocked off by passersby. I can go around the wall and, by knocking, tell 
by the hollow sound that some plaster didn't adhere/pulled away.

So we'll be removing everything hollow and redoing those parts. My question 
is, when plaster fails like this, is there a common culprit? Mix, 
application, treatment afterward, wall preparation? My notes don't give me 
clues as to why some parts would fail and some would not. Some elucidation 
would be appreciated. Thanks for your help!

Georgie
www.cobinthepark.ca