P4 Plugin currently will not start polling if the job is building or if concurrent builds are disabled. This prevents builds from being able to queue up, and creates a quiet period of at least the polling interval. This prevents the "continuous" part of continuous integration, need to remove this check to always allow polling to occur. Jobs configurations themselves can handle concurrency checks.
I am working on the merge request
EDIT: pull request created here: https://github.com/jenkinsci/p4-plugin/pull/79