Q: shared icloud calendar not syncing

a few months ago I started syncing all my calendars through icloud, and I gave read/write access to someone else via icloud.

this worked superb, if they set a calendar event in my calendar I'd get an alert and then it would appear in my calendar, this was great.

a week or two ago, it stopped working.

Any events that are they send to me do not trigger the alert any more and do not appear in my calendar in any way.

they do not appear in icloud.com, on the ios devices or on my mac any more (they used to).

If I set an event to the same calendar it appears in icloud.com and then in my ios clients and mac.

The calendar still shows them as authorized to "view & edit" my calendar, but some reason they can no longer edit or send me events as they could when this was setup.

Why has this stopped working and how can I get it working again as it was very useful.