Shared calendars are on one device but not another.

I have shared some calendars that I maintain in a Mac. For several months these calendars appeared normally on both the iPad 2 and iPhone 4S. Then something happened--we do not know what--and all the shared calendars dropped off the two iOS devices. From the Mac, then, I sent new invitations to these calendars, and the iOS user accepted them. Now these calendars appear on the iPad but not the iPhone.

I do not know if it matters, but the iPhone is still on 7.0.6, because the iPhone does not have enough available memory for the upgrade to 7.1.

Anyone have any guesses what the problem is? TIA