I ordered some Coopers Carbonation Drops from Amazon and will prolly use those for some of the batch. I'll still give the sugar a shot, too, though. I always boil water and create a solution when adding sugar to my beers, but I'm not sure what amount to pour/squirt into each grolsch bottle (1.5L, I think). I think I'll see if one of the carbonation drops works and if so, call it a day.
Not sure why they went flat. They are over a year old, so maybe that has something to do with it - no idea. Even flat they taste good and since it's coming up on IPA season, I want to drink/share them, so here's hoping this works! Thanks for all the help