Why don't my shared photos download properly to my iPad?

Here's an issue that has me scratching my head:

I am using all the latest versions of iPhoto on a MacBook running Yosemite, and the Photos app on a retina iPad running the latest iOS 8.1. (For a particular reason, the MacBook is not the Mac I use for regularly syncing my iPad.)

Thus, the standard way I transfer copies of some photos (e. g. to show to friends using the iPad) onto my iPad is via "iCloud Sharing" - I select the photos in question, then simply chose "Share using iCloud" in iPhoto and create a new photostream (e. g. "CPT photos"). Then the MacBook happily pushes the photos into the Cloud. So far so good.

When I then open the Photos app on my iPad (all on my own, fast home WiFi), and select the little cloud symbol marked "Shared", my shared collection of photos (e. g. "CPT photos") shows up as expected. It usually takes some time until all the photos have downloaded onto the iPad.

However, although the syncing process has stopped (no more spinning wheels anywhere), about half of the "shared" photos (a completely random sub-set, it seems) only show up as "pixellated preview mess" on my iPad, looking like this when I first open them on my iPad:

It then seems (as indicated by the " growing pie" infografic in the lower right hand corner), that - although the iPad has previously stopped syncing - it downloads high res versions of those as yet unsynced photos. (The iPad won't do this for the "preview mess" photos on its own - I have to look at each and every "shared" photo on the iPad to make sure it's downloaded properly. Which is a pain.)

And I don't have a clue why-oh-why the iPad doesn't download the high resolution versions of all shared photos in the first place?!

There is more than enough space on my iPad, and it cannot be a bandwidth problem either.

Any help or insight would be highly appreciated.