This seems to be one of the most frequently asked question besides "Do you workout" or "How much you bench". "I'm doing weighted crunches, slant board, knee raises and a few others but still having trouble carving out my abs".
Here's a novel idea. STOP doing ab exercises, and start working on your core as a whole system. A lot can be learned by taking a lesson from the Pilates playbook. Pilates mat classes will teach you an ab series unlike what you typically do in the gym, such as criss-cross or bicycle, 100s and other exercises that sculpt the abs and pull them in and up. Having an awareness around deep ab training, rather than just focusing on using weight in an effort to train heavy enough to get a six-pack is what probably needs to happen. Most people often stop at developing just the abs that show in a six-pack and don't work the intercostals and serratus, the soaz, or the obliques. Gym ab training typically doesn't really build strength in the core to support the lower back either.
Remember, your diet plays a huge part in how well your six-pack stands out. So eat smart.