Casein is difficult to re-suspend in shaker bottle. I use blender. Screw neighbors (I mean I prepare it before bed-time so it's quite late and my blender is loud). Two tips:
1. try adding couple of
ice cubes and then shake - they'll crash into larger clumps and break them
2. add twice as much liquid as you would for whey -
casein is thicker than whey and that's the reason for more clumps