depending on the alcohol content, time in secondary, temperature, etc....
1. I would pitch more fresh yeast in the bottling bucket
2. Make sure said yeast can tollerate the high alcohol
3. Give it a month mimimum to carbonate. 2 months would be great.
I had a issue with a 15% RIS and did not add any yeast. No carbonation after 6 months. opened the bottles, rehydrated some champagne yeast and use a syringe to add yeast to the bottles, re-capped and now it is carbonated correctly. This all is IMHO