Having Trouble Keeping Pygmy Cory's

[orangefish]

I have had 4 pygmy cory's in the last year and only 1 of them has survived, and they all appear to be mysterious deaths as there is no signs of illness and my water stats didn't appear out of place. I don't really get any deaths from the other fish in my tank either (touch wood!). Are pymgy cory's particularly sensitive to water? I have just been having no luck with them and they are really sweet fish.