The changelog of a build cannot be determined when a job is configured to "Execute concurrent builds if necessary".

When for example four builds run concurrent, three builds report
"WARN: Revision data for previous build unavailable; unable to determine change log"

One of the concurrent builds always is able to determine the changelog.
This is probably caused by the fact that the builds try to get the revision of the build which is executed concurrent and does not yet have the revision data determined.