[CentOS] update from local repo mirror

so this may be an odd question, but my google fu seems to be failing me.
 I've created a local mirror of centos 5 & 6 with various instructions
across the net, it seems to be functional, syncs on a regular basis,and is
generally usable (I've built machines with kickstart/netboot from it and
they seem generally workable).  my difficulty comes up when I try to use
the local mirror for system updates.

if I use the originally installed repo configs and run yum check-update, I
get a list of 10-15 RPMs for updates, if I change the repo files, do a yum
clean all, comment out mirrorlist and uncommenting/chang  baseurl=
http://localmirror/path/to/repo and run a check, I get nothing.
if I look in localmirror:/path/to/repo and do a find for the version
numbers I got as available updates from the remote repositories, I find
appropriate files in seemingly reasonably named directory paths.

my current best guess is that my repodata/repomd.xml file needs some sort
of update/rebuild via the createrepo script, but I can't seem to find the
'right' way to this.   does anyone have any pointers?

thanks.

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