You could leave it a while longer. Sounds like maybe the CO2 hasn't had enough time to go back into solution. Some of my beers, particularly the big ones, take longer. It looks like they're carbonated well at first, but they fizzle out pretty quickly. Some more conditioning time may very well take care of it.
Primary 1: 4 Hop IPA
Primary 2: Apfelwein
Primary 3: Air
Bottled: Hammered Squirrel Nut Brown Ale (btl. 5/9/10), Jamil's Evil Twin (btl. 3/29/10), Crazy Ivan RIS (btl. 2/14/10), English Pale Ale (btl. 12/29/09), Apfelwein