1.   possibly these users didn't get/send any email that qualifies for archiving.  how active are all your users?

2 & 3  the last time there was a change on their account?

4.  I wouldn't take that bet cause I've seen that happen as well, but haven't in so long I can't even find the bits needed to make a change.  What we need to do is to kick that LastItem time far enough on that user to get past the stuck message, then it moves along nicely.    This might be worth opening a support ticket, and if you do could you share the current steps for this if nobody else posts them before you do?       One of the fun bits is that we needed to calculate the unix time for the objects to kick it forward enough.

In the retain server web interface under "about" there is a button "Retention Date Utility" where you punch in the user and it shows you the values.  you can get and set the user's settings here as well.  I have never done it.  Our system is quite busy with mail and archives as I have scheduled tasks that run every night.

ahhh,  that's where it was hiding.  its been too long since I needed it.  They added showing the real world time since then, nice!

for those users in question,  what I had to do those years past was add a bit to the Item Store Flag.  So if the time is when they get lots of mail, perhaps add 20 seconds each day until it advances, or if at night perhaps advancing it by ~16 minute hunks by adding 1000

of course if you can track for sure the offending message, you could just go one second past it, which is where the offered (on that page) Online Unix timestamp converter helps.

that list you showed is new since I last had to do this myself, just haven't needed it since it was added.  looking closer on this system, the users without any "Advanced Date Flags" are inactive users, pending their removal from GroupWise.