iBooks - some books never load - just hang

I added iBooks to my sons iPad (1)

I went to my purchased books and attempted to download (from iCloud) the childrens books I had purchased and loaded on my iPad (mini)

The books NEVER download! - They just hang and even if I delete the app and all contents and start again it does the same thing every time!

I even plugged the iPad into my Macbook to sync by wire. - Same problem. Tried this for hours - no luck.

I tried to upload a grown up book and it worked... Both devices use the same apple ID

Very frustrating - any ideas?