yum security updates list not longer updating

I have a server that no longer lists the current security relevant updates when using:

yum list-security or yum --security check-updates

At one point yum tried to update a package and couldn't figure out how to resolve the dependencies. Since that time, new security updates aren't showing (I know there are some because I run other server with similar software packages). Yum shows that the only security relevant package is the one that failed. What steps can I take to get yum working again?

yum update still works on any package I manually tell it to update.