I made a really nice bourbon aged(cubes and willet) imperial stout in the spring that I had planned on giving out in 750ml bottles for Christmas gifts. Just opened one to test and it is un-carbonated. Totally flat. I have made the recipe before and did not have this issue. Each bottle was primed with 2 Muntons carb drops. Beer finished at 11.9%, I have heard of adding champagne yeast but as I said, exact same recipe 2 years ago worked great. Would warming them a bit help any? They have been stored at 65 degrees for 6 months. I have a rye stout that’s planned to finish at 7% about to finish in the primary could I try some slurry? Suggestions? I understand Christmas isn’t an option but could I do something and tell people not to open them for a couple weeks?