I'm in exactly the same situation. First it might help to have a look at the people network around the decision makers and see if you have someone in your personal network who is in more direct contact with one of them. A personal introduction always helps to gain trust.
Ofcourse TTN itself provides some nice promo material like:
- Video from Wienke
- Example use cases descriptions (I like the example of the port of Amsterdam as a nice example of "co-sponsorship")
Also there is some nice press coverage:
- Nieuwsuur: De opmars van het Internet der Dingen
- Recent article in Automatie
- Amsterdam
- How Groningen thinks about it
- Utrecht (Surfnet)
Probably others can add much more nice examples (maybe more geared towards building management)
To me it seems LoRaWAN is not always the best technology for building management because other wireless (and wired) technologies exist that are already used succesfully for that purpose.
I guess you should make them somehow feel like they are missing out when they don't participate 
Oh, and you might want to show them your ttnmapper.org site to show how they can help to increase coverage (unless the building is in the center of Enschede, then better not show the map 
)