Given this repo structure, where t2 was the last revision that was built by jenkins, when t3 is pushed to the central repo, polling fails and says that no changes have occurred.
I think this is because after polling pulls the specified branch, it then only checks for changes on that branch excluding any merges, using "hg log --branch trunk --no-merges". This returns nothing in the above scenario, so no build occurs. If either the --branch option is removed it returns d2 & d1 changesets or if the --no-merges option is removed it returns the merge changeset, so I assume will perform a build.
Performing the build manually with the Build Now button works correctly, and shows the correct changesets that were involved.