Assistant professor does not collaborate with or cite fellow department members, despite them being experts on the topic
You seem to think that an assistant professor should collaborate within your department.  However, a more common view is that an assistant professor should be an independent researcher.  An assistant professor should clearly distinguish their research from that of other faculty at the same institution.  Assistant professors do not want to be asked, "Why should we grant you tenure when we already have a tenured professor who can do the same research?"  

Without more detail added to the question, it sounds like the assistant professor's collaboration strategy is okay.