I always try to make screenshots of running apps, with a 70% chance that it won't work. There is a green light showing my connected device. Most of the times I have to connect and disconnect like 10 times until it can make a screenshot.
What causes this problem? Must I wait 10 minutes after build? Is there any logic behind this failure?
In my own experience its completely broken. You may have different results, but using the version of Xcode that ships with the 3.2 SDK I cant take screenshots of the device at all. I' have resorted to using the version of xcode that ships with the 4.0 beta, since screenshots work in that version.
I think you'll just have I wait until they fix it, or do as I do and use the beta just for screenshots.