Ribbon design in CRM 2011 can be quite painful. It’s not documented that well in the SDK, and browsing through the ribbon XML is a real trivial task.
Recently I was trying to add a custom tab to the campaign entity in CRM. This tab should contain 3 groups which contained several buttons.
Following the guides posted here by Siva (http://dynamicscrm2011.wordpress.com/category/crm-2011-ribbon-customization/), I was able to do a great deal of the ribbon design. The only problem here was that he used only one group in the ribbon, which also worked for me.
As soon as I wanted to add a second and third group, no buttons were displayed for these groups.
It turned out that the reason for this was that I needed to define a MaxSize in the Tab node for each of these groups.
You will also need to do the same thing for the scaling of each group of course.
Check the “Define how Ribbon elements are re-sized based on the space available” part of the SDK for more info on this.