Any chance you could share some sample data, even if it's just a mockup?
If there is any null may be that's why you are getting blank
(SUM(ZN([1-5]))+SUM(ZN([6 - 15] ))+SUM(ZN([16 - 30]))+SUM(ZN([30+])))/SUM(Number of Records)
Hope this helps kindly mark this answer as correct/helpful so that it will help others
The ZN seemed to be the problem although it seems that using Number of Records didn't work for me. I ended up making a different calc called Open Tickets and then using that instead of Number of Records in the table. So by using ZN and the new Open Tickets it works now.
SUM(ZN([1 - 5]) + ZN([6 - 15]) + ZN([16 - 30]) + ZN([30+]))/ SUM([Open Tickets])