Q: downloads don't sync

I downloaded a couple 'sample' books from the bookstore onto my iMac.  However, I discovered that those books did not automatically download to my iPad Air or my MacBook Pro even though all three devices are running on the same Apple account.

I had to manually download the same books to the other devices, and even now, the pages won't sync.

This is the first time I've had this problem, so I'm not sure what's wrong or how to fix it.

Any suggestions?

Thanks.