Another bug in the Solution Explorer window that was reported in the forums some time ago:

BUG: EnvDTE.SelectionEvents.OnChange event does not fire when more than two nodes are already selected in the Solution Explorer of Visual Studio
http://www.mztools.com/articles/2007/MZ022.htm

I reported it to the Microsoft Connect site so you can follow the resolution:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=269532