Fixing this WAY undercarbbed Russian Imperial

So this is my first actual post but I've been a long time follower of HBT. Truly an amazing community of home brewers.
So I brewed a big RIS back in April and bottled it in July and cracked the first one in October. I added an entire vial of the same White Labs yeast I originally fermented with to the secondary a few days before bottling. Definitely added my priming sugar to the bottling bucket. The bottles have been at proper temps since bottling day. (70-75) I have occasionally opened bottles to see if carbonation is changing at all and have even shaken every bottle multiple times. Im at a total loss for answers and I haven't found a solid answer on the forums for actual steps to fix an undercarbed beer. I am now thinking maybe because its such a big beer it would require more priming sugar? (I just used the standard 5 oz bag ) But from what I understand it would of still eventually carbonated even with to little priming sugar. The carbonation is SEVERELY lacking ..its almost flat ..just a tiny tiny bite on the tongue and a non existent head. I have had succes witha prior batch injecting 2ML of a yeast mixture into each bottle and recapping but that batch was completely flat. I am not sure if you can succesfully do this with an already somewhat carbbed beer. Any and all ideas would be greatly appreciated.

No idea why it didn't carbonate, but hey, at least it's a stout.

1. Pour beer in a glass.
2. Steam some milk.
3. Let it settle.
4. Scoop the foam off the milk.
5. Add foam to beer.
6. Profit.

I'm having some success putting flat beer in a 2 liter plastic bottle and injecting CO2 into it. Let it sit in refer for several days and drink.

I've no idea why it didn't carbonate in the first place though.

as long as there is viable yeast still in the bottles (that is, it's an undercarbing issue and not no carbing):

rouse the yeast by turning upside down for 3 days, then back rightside up for 3 days, all at your bottle conditioning temp

my very first brew was undercarbed and this cured it.

I've found for some reason or another my big RIS brews never carb in the bottle even with the addition of WLP099 or champagne yeast. Then again I let mine age for at least 8-9 months with a brett culture. For whatever reason even with addition of a proper amount of priming sugar and 4-5 months of conditioning time I have two years that never carbed. So I have resulted to realizing I need to force carb and bottle via beer gun. I have to dump my most recently bottled batch under CO2 and force carb.

I had something similar happen with a BDSA at 12%. I fixed it by squirting in a bit of nutrient and yeast slurry mixture. I guess the yeast were too tired after the big ferment. You added fresh yeast, though, so you should be fine.

I had the same problem with a RIS. Tried rousing the yeast, warming it up, with no luck. I guess I should be thankful it was only a 3 gallon batch. I am currently using it to cook with. It makes a damn fine chili or beef stew, though!

I read somewhere (maybe it was Wild Brews?) that bulk aging for long enough will allow enough of the CO2 to leave solution that even after the yeast in the bottle eat all of the priming sugar, the beer doesn't seem carbonated. The solution as I recall was to add a couple ounces of sugar to the carboy about a week before bottling.

Anyone come across this datum and can confirm where I might have heard it?

What was the OG and FG? How much priming sugar did you use and at what temperature have the bottles been sitting?

Something isn't right, especially if you added yeast at bottling.....

Edit:just re-read, temp is good and assuming a 5 gal batch amount of sugar should be good as well.

When you open the bottle do you hear a little hiss? If so and you stated you're getting a little bite then they just need more time. Some really big beers can take forever.