Are you doing it by antisolvent method? may be too rapid. Dissolve in minimum amount of solvent at an elevated temp. Cool the solution slowly. You may need to hold the solution at a temp for a long time if its not crystallizing by primary nucleation. If you know the solubility cool to just below the solubility and hold, try scratching the solution with a sharp object to encourage heterogeneous nucleation. Or leave the sharp object suspended so it has something to grab onto when it decides to go. When it does go the nucleation rate will be low because the supersaturation is low, then you're more likely to grow onto existing nuclei, rather than forming many nuclei (powder).