[CentOS] quota doesn't appear to work - repquota only updates when quotacheck is run

I have set up user quotas on an ext4 filesystem.  It does not appear that
the quota system is being updated, except when I manually run quotacheck.

More detail: I run "warnquota -s" from a script in /etc/cron.daily.  I
noticed that no one had received an "over quota" message in a long time.
 Using "repquota -as", it indeed looked as though everyone was under their
quotas.  But "du" showed many people were over quota.  So I did a "quotaoff
-a ; quotacheck -vam ; quotaon -a".  That night, several
warnquota-generated messages went out.  My users diligently cleaned up
their homes.  Fast forward 24 hours, and the users received the same
warnquota emails.  repquota showed them as being over, but "du" told a
different story.

System is CentOS 6.3, kernel 2.6.32-279.2.1.el6.x86_64.

# dmesg | grep -i quota
VFS: Disk quotas dquot_6.5.2


The partition is type ext4 mounted at /share:

# cat /proc/mounts | grep share
/dev/mapper/VolGroup_Share-LogVol_Share /share ext4
rw,noatime,nodiratime,barrier=0,nobh,data=writeback,jqfmt=vfsv0,usrjquota=aquota.user
0 0

The ext4 volume sits on top of an lvm logical partion.  That logical volume
ultimately sits on top of an encrypted disk using cryptsetup luksFormat:

# lvscan
  ACTIVE            '/dev/VolGroup_Share/LogVol_Share' [4.48 TiB] inherit

# pvscan
  PV /dev/mapper/luks-7f865362-ee9f-40de-bc07-73701b4662f3   VG
VolGroup_Share   lvm2 [4.48 TiB / 0    free]

Is there something in my ext4 mount options that is incompatible with
quota?  Or maybe the encrypted layer is causing problems?  Am I missing
something else?

Thanks!