The most likely problem is that the priming sugar was not thoroughly mixed into the beer before bottling.
No...more than likely if a few are now carbed, the rest just haven't come online yet. They are each in their own little yeastie "eco-systems" so they all have their own time frames.
You might consider rolling the bottles once on their side to re-suspend the yeast and make sure the temp is 70 or above...and they will come to carb eventually.
If it's 6 weeks, betcha by 8 they will be fine. If you read the blog I linked you'll see I've had stouts and porters take 8 weeks, but they carbed eventually.
Heck even in Old English brewing books, they
don't add any priming sugar at all in some lower grav beers, and let the co2 build up
over time...of course it's not like the highly carbed beer we americans were raised on, but they are at the lower end of our carbontation range if we use software. In fact I ran the numbers of some of those recipes in beersmith and some of the numbers actually came up in the negative for amount of sugar added...meaning you didn't need to add any sugar at all, you just had to let them sit and they would reach the mid range of carbonation...with patience.
So more than likely if you agitate the yeast make sure the temp is above 70 and walk away for a bot longer, they will all starting "popping."