The plugins waits previous build to complete without having the need to manipulate data from these. We do use the option to build same project in parallel, and this caused serialization of builds.
This is happening because of the method getRequiredMonitorService returns BuildStepMonitor.BUILD (https://github.com/jenkinsci/naginator-plugin/blob/master/src/main/java/com/chikli/hudson/plugin/naginator/NaginatorPublisher.java#L87). Could it be changed to return BuildStepMonitor.NONE?