Thank you everyone for your replies.  The problem has been solved, but the
information you have all provided is invaluable to me
in understanding other potential issues I should investigate, or may
encounter, with ADSM.  I am keeping it on hand.

The problem is that when tapes are being checked in, they are not being
properly changed from vaultretrieve to onsiteretrieve
status.  Oddly, it seems to do a few, but not all.  I'm still investigating
this.

I have recovered almost 100 tapes now with:

move drmedia "*" wherestate=vaultretrieve tostate=onsiteretrieve
checkin libv libname search=yes status=scr

Thanks again folks,
Dan.