We experience the same issue in our environment using version 1.1.1
The artifacts are managed by Artifactory. The repository is configured with Maven Snapshot Version Behavior: Nonunique
Therefore SNAPSHOT artifacts are always stored using the same file name.
Artifactory documentation: http://www.jfrog.com/confluence/display/RTF/Local+Repositories#LocalRepositories-MavenSnapshotVersionBehavior
I've debugged the plugin and found out that it checks if the artifact is available in the local cache and if so, no further checks regarding the last update time are made. Therefore the locally cached file of a SNAPSHOT artifact is never updated.
As a workaround, we have now configured the repository to use unique file names for SNAPSHOT artifacts. The filename now changes every time a new version of the artifact is deployed. Of course the new file is never available in the local cache and therefore the plugin downloads the artifact from the repository.