I had a similar problem, that took me ages to solve. At the end of the day it was just me being stupid, but thought I'd share the issue so others facing same issue can try it.
My scenario was a PopupControlContainerTool, and previously it worked fine. I then did some fairly major rework and it would never become enabled. Eventually I realised I was forgetting to set the Control that the tool contains, kind of obvious really :)
Is it a menu tool with all disabled child tools? If not, is it possible it because hidden at run-time? Hidden ribbon tools will actually disable instead of hiding if Office2007UICompatibility to True. If neither of these is true, it is possible there is a bug and I would recommend submitting the issue to the support group: http://es.infragistics.com/gethelp.