How do I cook meat in the crock pot? ?

Everytime I put something in the crock, it comes out tough! I'm not overcooking, I keep in on low for 4-6 hours and it's still not right. I've tried all cuts of beef, and now I bought a pork to try. I want it to break apart with a fork and melt in my mouth. Also, any coconut/curry ideas for the crock will be greatly appreciated! Thank you!