"Hard" sounds, ones where you need to "force" or "push" the sound through are the ones that I often block on. I start to feel the block and my natural reaction is to apply more pressure, trying to force the sound out. This, of course, only makes it worse. When you're stressed out, struggling with something and embarassed on top of it and the solution is to "just relax", it's difficult! This is where I find active practice of my techniques comes into play, and the more pro-active in my practicing, the more likely I am to remember to "stop forcing".