Have you boiled it? Is it attached to a piece of slate? I think the time will be different for each piece of driftwood. Sometimes you can place it in the tank and weigh it down for a few days before it stays on the bottom by itself.
hi. i was told by my aquatic shop that ideally you should soak it water for at least 3 days , then discard the water and soak for another 24 hrs in clean boiling water just to get rid of the remaining tannins so that when you place it in your tank it shouldnt make your clean water go mirky. my drifwood was soaked for this amount of time and placed in tank and settled nicely at bottom without being weighed down.