high-alcohol beer is a stressful environment for yeast. as alcohol % rises, yeast starts to die off - essentially choking on their own waste by-products (alcohol). some manage to hang in there, go dormant, etc. by the time you add your bottling sugar there just isn't as much active yeast in there as there was at the peak of fermentation, or in a post-fermentation lower-gravity beer. the yeasties that are left will get around to digesting your bottling sugar eventually. they'll just need more time, since there are less of them to get the job done. that's why you might add new yeast before bottling an aged high-gravity beer: to make up for the lack of primary yeast.What's the reasoning behind big beers taking longer to carb up? I have one IIPA that is taking a while right now and am curious.
one thing that i've found that helps with carbonation is to occasionally turn the bottles upside-down and gently swirl to get the yeast back in suspension. this increases the chances that the yeast will encounter sugar molecules.