are there any solvents which solubilise it more? In any case, if you can hold at elevated temps, you can choose the largest crystal, tie it with a thread and suspend it in the supersaturated solution. Even if there are smaller crystals still in the bottom, the larger suspended crystal will still grow. Look up Ostwalds ripening. But it is better when the solution is free from other crystals so filtering the solution into a clean container every so often will help with the growth.