photostream not working

have upgraded to iOS 7 on my iPhone 4s. since then, photostream seems to not be working most of the time. photos taken with the camera app do not show up in the My Photo Stream album in the Photos app and they do not show up in Aperture on my Mac. all settings are correctly set. earlier, i found a thread with similar issues, but i can't find it again.

how do i resolve this?