Have you brewed a mild before? When did you keg it? What yeast did you use? Recipe?
I'm beginning to believe that milds - I've brewed 3-4 of them now - really are at their peak young and need to be consumed young to get at their best. Some people have reported that english strains of yeast change character of the beer depending on how/when they are handled and packaged. It's quite possible for the character of a beer to disappear and then reappear later, strange as it may be.
I know that with my last mild, the malt/sugar character was best within a month of brewday, then it declined. The one thing I have to account for is oxidation during transfers and aging. This could also make a beer seem lifeless rather quickly.
Bottled: Old Peculier, Kumquat Kolsch, Dopplebock
Kegs: Helles, Oktoberfest, Hefeweizen
On Deck: English Bitter, RIS