Priming has nothing to do with AG vs extract brewing. You could carbonate water with that sugar as long as there was yeast present.
Three things come to mind:
-Is enough yeast getting into the bottles? If the yeast has settled a lot, try dragging the racking cane/autosiphon along he bottom to get some yeast back into suspension.
-Are you storing the bottles in a warm enough place? People might just store bottles in the fridge, but the yeast isn't going to do a whole lot at those temperatures.
-Are you letting the bottles chill for a while before drinking? 48 hours is a really good amount of time. The CO2 that the yeast produces largely remains in the headspace until it's cooled, and even then it's not instantaneously dissolved into the beer, it needs a bit of time. In particular, the practice of sticking bottles in the freezer to quickly bring beer to a drinkable temperature is an extreme example of this, but even sticking it in the fridge for just enough time to get cold enough is going to result in the beer itself being less carbonated than it should be, even though there is an appropriate amount of CO2 in the bottle overall.
Oh, and make sure the sugar is mixed into the beer VERY WELL before bottling. If the above three things don't appear to work, it's probably this, and can be demonstrated by using carbonation drops in each individual bottle instead.