Give it more time before you make any changes. Bottling without adding more yeast, that close to the tolerance of the yeast will make carbonation take longer. Adding fresh yeast, to something with that high of an ABV (already) can be tricky. You can shock the yeast that you add, making it take longer to carbonate. You can also have bottle bombs due to wine yeast eating sugars that the beer yeast won't.
Wyeast 1728 can go beyond 12%. Since you have SOME carbonation already, just give it more TIME. Six months (or more) is NOT uncommon in bigger brews.
BTW, for the next batch, consider carbonating on CO2 (in keg). It really is the only way to get carbonation closer to your time frame.