I just bought an iPad mini. Some of the photos were downloaded from my iPhone but not all. Why?

Photo stream photos only remain in iCloud for 30 days, giving your devices enough time to download them. When you enabled photo stream on your iPad mini, you only downloaded photos from the last 30 days, as older photos are no longer in iCloud. To stream older photos to your iPad, add them to a shared stream on your phone (http://help.apple.com/icloud/#/mmc0cd7e99). Then you can save them to the camera roll on your iPad mini and delete the shared stream.

Probably, you need to get iCloud and hook up your iPhone and then copy them on to your computer, and then connect your iPad Mini and then download the photo(s) from there.

I had the same problem. If this does not work, reboot:

Press and hold Power and Home buttons for 15 seconds, ignore the slider and wait for the apple to appear.

IT takes a few days. But they should all download.