Searching didn’t help, because most recarbonate topics were for an entire batch.
I made a batch, all bottles, with one growler.
The bottles are good, normal beer. Ok, not normal… great beer!! 
But my growler is flat. I blame the plastic cap. Although my last batch with the same growler was fine. It may have a slow leak, as I opened this growler months after making this beer.
Is there a safe way to recarbonate? The beer tastes ok, it’s just flat. Again, the growler was bottled the same time as my other bottles, so I know it originally SHOULD have carbonated as normal.
Should I try to add some sugar? I don’t have any Coopers or Muntons tabs.
Or should I let it go and chalk it up to experience?
I should mention it was fermenting for probably 3 or 4 months before I stuck it in the fridge. I planned on using it earlier for a neighborhood gathering, but it got rained out. It’s been in the fridge for 2 months or so.
I use white table sugar or cane sugar to prime all of my beers, you can try to recarbonate the growler. Your issue is a possible leak in the cap, if your cap is leaking then what is the point of trying to keep carbonating it. I think i would just forget it, i wouldn’t wait the extra couple of weeks to find out if it did or did not carbonate.
It’s only 1 growler and I would live with it and not use that growler again.
Any sediment in the bottom? Just thinking if not it may have never carbonated in the first place. If the top of the growler isn’t chipped or something I would also suspect the cap. Most homebrew shops sell them in either plastic or metal with a rubber seal built in.
I would toss in a small amount of table sugar and give it a try, drink it flat or use it to cook with.
I would get two 1L PET bottles of club soda, pour out the soda, add two teaspoons of dry sugar and a tiny amount of dry yeast to each bottle, then carefully and slowly pour the cold beer out of the growler into the bottles, holding the bottles almost horizontal to start and then slowly tilting them upright as they fill. Stop when the beer is about an inch from the top, then squeeze the sides of the bottle to push the beer up to the lip and screw on the cap. Then invert a couple times to dissolve the sugar and store at 75-80F for a couple of weeks.
