Mapshare track not updating X 9 hours?

There isn't a simple answer. Since the send interval neatly changed from every 2 hours to every 4 hours, that suggests your family member changed the send interval.  Perhaps he noticed his battery was draining faster than expected and changed the interval to preserve the battery. With nothing since, his battery could be dead. The device may have also been dropped in an irretrievable place.

I would send a message to Mapshare recipients if I was going to change the tracking interval. Did you check your email account or phone text messages to see if he sent you a message there? He could also send to Mapshare, but I'm assuming he did not.

Do you know what device he has and do you know whether the battery was fully charged?

Hiking under a heavy tree canopy or rock outcroppings or within gulleys can prevent signal transmission. The device will continue to try to send and then time out. This tends to drain the battery faster than when hiking in more open environments and the tracking point is quickly sent.

Garmin posts the status of inReach and everything seems fine (though there can be reporting delays): https://status.inreach.garmin.com/

You can also go to www.iridiumwhere.com and zoom into the map where he is hiking. Tap onto that location and you will see the current satellite coverage. If you view for awhile you can see that the satellites move in lanes and the number of satellites with coverage for that location will change. When a single satellite is very close or when two or more satellites are somewhat close, you can expect transmission to be good. If his battery is not dead, you should expect to receive a tracking point then and you can send him a message (note that when you send a message, it's not like a cell phone, there can be a long delay).

You should be concerned, but the level of concern should be commensurate with the experience level of the hikers, are they hiking where other hikers would be expected, the terrain and wildlife where they are, when they are expected to return, etc.

very helpful - thank you!  Tracking still has not updated since 4 am, but I have been able to reach them and all is fine. Seems like it must be some technical issue that we'll need to resolve when they return.  

The other thing is that the unit will automatically fall back to 4 hour intervals when it is stationary. This is probably what happened knitially sknce the four-hour intervals started around 8PM - presumably when  they stopped for the night. Once the unit decides that it is stationary, it checks for a "sufficiently large" distance at the original interval (which is 2 hours in this case). This means that the unit might not resume 2-hour interval tracking until 2 hours AFTER the unit moves. This does not account for complete lack of tracking the next day, however.

"Locate" from the web site is a crap shoot at best. The problem is that the unit has to "hear" the locate request in order to respond. Must be powered up, have a clear sky view, AND do an active listen (as, for example, when sending a point, sending a message, or the hourly message check. In my experience, this almost NEVER works. And with a 2 hour tracking interval, your chances of hitting a check with the locate request are very low.

I would not necessarily dismiss "dead battery" out of hand. The unit will perform an active message check once an hour. Once it starts the process, it (re)tries VERY hard to complete it. It's not "complete" until the unit contacts the Iridium satellite network. So - if you bury the unit in your pack, or if you put it under your pillow in your tent, or whatever - you can deplete the battery quickly. You don't say what model unit this is, so it's difficult to judge battery life.

Most of use turn the unit OFF when we stop for the night. The unit will send one more "tracking end" point at that time.

Looking at Iridium satellite coverage probably isn't useful. Iridium gives pole to pole coverage. I don't recall the details, but there are usually 2 or 3 satellites in view at any give location/time. (Some may be low on the horizon, though.) Point is that the satellites are THERE. The two primary enemies of iR communication are narrow canyons and heavy foliage (particularly wet foliage). Note that the unit queues sent points if it cannot send them in a timely manner. There is a limit to how many points it will queue, but you are really, really unlikely to hit that with a send interval of 2 hours. In any case, if you do hit the limit, the unit discards the oldest points first. It's bot like you are going to lose all of them.

Do note that the unit will NOT send (or queue) sent points if it cannot get a GPS fix. If this is a mini (particularly M1), you cannot discount that problem as a source of missing points.