PopUp Pro - pop up not showing consistently

I have a pop up set up to show on a specific page and only to logged in users. On my development site it works perfectly however, on the live site - which is on a different server - it occasionally stops showing at all.

The sites are identical - other than the server they are hosted on.

I tried turning off all the other plugins to see what the deal was - and it appears to have been a conflict with the iThemes Security Pro plugin. When I turned it off, the pop-up reappeared. However, when I turned it on again the pop up continued to work correctly without me having changed any settings.

I am just trying to figure out what the problem might be so that I can reassure my client that his site is not going to have an intermittent error . The pop up is currently working - but I am not sure how to troubleshoot further to figure what stopped it in the first place and would appreciate any assistance you can offer

Cheers
Meg