My best guess would be that your yeast was dead (or dropped out of suspension) when you bottled, because 2 carb drops should have given you plenty of carbonation. Can you see any yeast sediment in the bottom of each bottle?
Are they PET bottles? You could try opening each bottle and adding some yeast, then sealing them back up again. A few ways to do it, randomly sprinkle some dry yeast in each, or re-hydrate some dry yeast, shake it up in a jar of some sort, then use a sanitised pipette or syringe (i'd go pipette) to squirt some into each bottle.
What temp have you been storing the bottles at? It just struck me you might have them too cold (in which case the yeast is asleep), or too hot (in which case it may have killed the yeast).