This is a (somewhat) obscure issue that has been driving us nuts for months now, but we finally seem to have narrowed it down. The issue is that when we used the RibbonView as a Prism region and activated RibbonTabs in that region, the RibbonTab would occasionally fail to render. What I mean by "fail to render" is that the RibbonTab would get added, but would be completely empty; just a blank, grey tab.
We fought this for months assuming that we had a bug somewhere in our region management code. The problem even went away for some unknown reason, but came back just this week. After scratching our heads for a while, we figured out that if we set IsAutoSize=False on all our RadRibbon elements and made sure to set Size=<somevalue> then the problem would go away. If we tried to use auto-sizing then the problem would come back and even get worse as we made more and more elements use auto-sizing.
This appears to be a similar issue: http://www.telerik.com/community/forums/wpf/ribbonview/ribbongroups-not-visible-issue.aspx
So the good news is that we figured out a viable work-around. The bad news is that there appears
to be an issue with how size is calculated. I'm sure the folks at Telerik can whip up their own sample to see what's happening. Meanwhile, we're just glad to have identified a solution that will let us roll into production.