Hello, I'm not far from Toronto :-)
We had the same problem , one year the Sandcherry was gorgious, beautiful little
flowers on it, then it got sick.....
in our case it turned out to be grubs in the ground, so hubby put some pellets in the ground to kill the grubs, it is not producing much
flower, however it has not died, and the little foilage that it does have, is staying and the Sandcherry itslef is healthier looking...
I also read that Sandcherry's don't have a very long life span too though ... so I guess we will have to wait and see next summer how well it does