its all luck maybe monster specific cap aka this monster wont drop for this many hours etc or some1 else there got them instead of u, they dont do player caps.
*Sigh* there's NO SUCH THING as a cap. It's all luck. Every time you kill an abby demon, there's a 1/512 chance it will drop a whip (confirmed by a jmod). You can go thousands of kills without getting one (in your case), or you can get one every hundred kills. It's all chance.