Soaking for sometime usually gets them to sing but attatching them to bits of slate (whic can be hidden under the gravel) and maybe even try attatching some aquatic plant lead stips to it.
i find that usually after they've been boiled for a while to take out all the tanins they will sink after being put into the tank with little to no assistance. If that doesnt work try holding it down with something for a while, if its been boiled and all that it should sink before your next water change