Cannot sync photos between two iPads

Hello, members of Apple Community

I have a little hiccup and would like to have some great advice

My iPad Mini 4 (iOS 9.3.2) and my grandma's iPad 2 (iOS 9.2.1) share the same Apple ID. I used to own an iPad Air, the Air and the iPad 2 sync photos flawlessly. But when I switch to the new iPad Mini 4, it doesn't update my photo library from the Air. When I take a photo, it still appears on the iPad 2, but the Mini 4 doesn't sync all the old photos on the iPad 2.

What happened? Is it because of different iOS versions?

Thanks.