Good question. On http://www.winespectator.com, Dr. Vinny recently answered that question by saying the following:
"A wine going through a 'dumb' phase is going through an awkward, transitional period as the wine evolves from youth to maturity. It mostly occurs in red wines. As a wine's fruit flavors start to decrease and the mature complexities of an older wine start to develop, the declining fruit and emerging complexity can cancel each other out. The wine will taste muted, you can barely make out the fruit flavors, and it just won't do much for you.
The cause is uncertain, and the time frame varies. If you have a case of something that you suspect is going through a dumb phase in your cellar, all you can do is sigh and wait another six months to a year before opening another bottle."
It's not common for 11-yr old wines to be going through a dumb phase. Another cause for muted aromas and flavors is TCA (corked wine). At very low levels of concentration, the characteristic mustiness can be difficult to detect; instead, the wine seems muted or dumb.