My wife has a SodaStream device made for infusing carbonation into H2O for bubbly water. We just wondered. We tried it. Let me tell you, this is the solution to a problem I hope never to have again. One blast from the CO2 bottle and I had wonderful head and fizz. Forced carbonation baby step. I have no keg or CO2 bottle, and it wouldn't have helped in the bottle, anyway. This is a solution for those in that unfortunate position.
I have been wondering how well this works. I brought a growler to a friends house that was flat. I'm still having issues bottling from keg to growler or bottle. They had a sodastream but no co2 canister so I never got to try it. I'm glad to hear it worked.
I have a bottled Christmas ale that has been slow to carbonate, the same amount of time as yours. I gave most of them away knowing they might not be fully carbonated by Christmas and told people to wait a week or so before drinking them. The ones I have opened recently were still flat. I used the same process I always do, which is pretty much the same thing you did and others suggested, and for some reason they werent carbonated. I can only think that the temps in our house were low and slowed the yeast down...or there is a problem with the yeast. Still tastes good. I might have to check one tonight.