When bottling homebrew, carbonation occurs by giving the yeast a small amount of extra priming sugar to consume. After capping, the yeast will eat the sugar and produce CO2, but since the bottle is sealed the carbonation will have nowhere to go but into solution in the beer. In order for the yeast to do their job, they have to have ideal conditions, in particular, a temperature range that’s warm enough for them to work. This usually means about 21˚C. If the beer is stored in a cold basement or refrigerator during this phase, the yeast won’t carbonate the beer or may do so very slowly. Make sure the bottles are in a warm room and give them some more time.