Photos doesn't sync from iPhone 5S to iPad 2

I have an iPad 2 and an iPhone 5S. I use Photo Stream but only some of the photos that I take on the iPhone appears on the iPad. It seems random which photos sync. When I look at the iPad I can see photos from 5 months ago, I can also find various photos from all the months until today - but not all of them. I tried to turn off Photo Stream on the iPad, delete photos from the photostream, then turn it on again but it just doesn't work! Any ideas what could be wrong?