Adding Apps to a meeting (Sometimes can, sometimes can't)

Hi folks.  My team has successfully released our first MST app but we have been encountering a problem lately that we don't know how to get around.  It seems that sometimes the user is not able to add any apps to a call/meeting in the native MST app, and other times they can.  We've not been able to find the common thread between when they can't or can't.  We have some people that have never been able to, and we have some that have experienced the issue more frequently than others.  We are in a QA environment so we are all using the same meetings to do our testing.  

Any ideas?  Thanks in advance.