% of one product against the other not working

Hi all,

Once again thanks for all your continued help ;-)

I have a pivot table with

Sum of Total Enrolled Devices
Sum of total Active Devices2

I have both the sums in the value filed in the pivot table and am trying to work out the % of Active Devices against Enrolled Devices for each month as per the table attatched.

In the pivot I have right clicked on Active Devices and chosen Show Value as % of and clicked on Enrolled Devices in the drop down box. However the value should be 83% but this isnt showing? I have tried and worked it out but its not working? Am i missing something obvious?

Would really appreciate your help as this is something which am struggling with but thought was straightforward!!