12% beers are difficult to carbonate using the standard method. You pretty much need actively fermenting, alcohol-tolerant yeast like WLP 099 to get the job done. At this point, your options are limited. Making a healthy starter of WLP 099 and eye-dropping some into each bottle may be your best bet. Or kegging under a CO2 blanket, force-carbonating and beer-gunning the beer back into bottles.
Other options are more risky - Brett, enzymes, etc.