Michael: My module is on a TAG. The project polls hourly for changes on a tag named RTB_13_4. I can probably provide a config or log if needed. I also looked at the code a little, but wasn't sure where to start.

Searching for changes on a tag in a multi-module setup is not as easy at it sounds. Consider the following:
1. A user checks in changes on a branch/head (we may or may not be monitoring this location, it doesn't really matter). At the same time they check in changes on a branch/head we are monitoring.
2. Polling picks up these changes and a build is scheduled
3. The build completes so the last checked timestamp is rolled forward to the build's start time.
4. The user changes the existing tag to point to a new version of the file. Since tagging doesn't create any commit messages and the commit messages for the changed files have already been seen in the last build, no message appears in rlog for the files changed in the tag. Polling therefore doesn't see a change so doesn't schedule a build.

It would be possible to pick-up changes on a tag if the changes and re-tag happen in the same polling cycle, but there's no way to guarantee this especially if polling is scheduled to happen frequently. I can apply such a change before the next release, although this could result in users who change tag contents seeing inconsistent polling behaviour depending on how their commits compare to the polling cycle.

To try and pick-up all situations where a tag would change, polling would have to be done using the full repository history - not just changes between the last build and current poll. This isn't particularly practical as frequently changed repositories are going to have a large change-log so require a lot of time, memory and processing cycles to retrieve and parse.

Code changed in jenkins
User: Michael Clarke
Path:
src/main/java/hudson/scm/CvsLog.java
src/test/java/hudson/scm/CvsChangeLogHelperTest.java
src/test/java/hudson/scm/CvsLogTest.java
http://jenkins-ci.org/commit/cvs-plugin/1900841791d1fbafc1839f0adea99452718f65a8
Log:
JENKINS-14711 Pick up changes on a tag since last polling cycle

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Snapshot build of changes at https://buildhive.cloudbees.com/job/jenkinsci/job/cvs-plugin/80/org.jenkins-ci.plugins$cvs/artifact/org.jenkins-ci.plugins/cvs/2.9-SNAPSHOT/cvs-2.9-SNAPSHOT.hpi

These changes are not currently on head so will not be on the next release until I've had more of a think about this and someone has convinced me that we should poll for tag changes

Given my previous comment about the risks in trying to pick changes in a tag, I have decided that this issue will not be fixed. Moving a tag is regarded as bad practice and trying to introduce a method of finding these changes would significantly increase CPU and Memory usage of the CVS plugin which such a fix doesn't warrant.