Coffee Stout

That's at least one reason (may be others, too)

 

The unfermentable sugars are there to stay. It won't get less sweet with time.
That said, the perception of sweetness will be reduced somewhat by the carbonic acid from the CO2. I'd give it a few more weeks before I'd pass judgment on this beer.

 

I usually test one bottle at about 8-9 days and mine have thusfar ALWAYS been mostly carbed (except for one batch where I added only 1/4 of the needed priming sugar, but that was a long time ago and the mistake was pretty obvious). That's not to say they're not still pretty green at 8-9 days, but the carbonation is always there (I just like to pop a test bottle, lol, to see how my "wares" are coming along). After about 3 weeks they're usually way perfect.

Note: I've been drinking them pretty fast, and have thus been using a little more priming sugar than most people here recommend. I've been using the full 5 oz pack per 5 gallons. I know you can cut it to four, and that's more "optimal." However, we've been drinking them so fast that bottle bombs are not an issue, and none of them get over-carbed because they get drank so fast once they're ready.

Now for the few of my beers which have been left longer than a couple months at this priming rate, they are pretty damn carbonated, borderline over-carbed. So take the advice of those who are saying 4oz/5gal priming sugar ratio seriously. And remember, this is for beers that have fully finished fermenting and don't have any residual fermentable sugars when you bottle!

Also, note that I've kept the primed bottles at a very reasonable temperature (near 70F or even a little higher). Colder than this and they won't carb up as fast.

oh and I always weigh the sugar on a gram scale, even if it comes in a supposed "5 oz" package, just to make sure I'm accurate on the amounts.