Those extra hard blocks are tough. My trick is to just stop trying for a little bit. I can always feel those hard blocks in my mind and body and I can feel when they go away. The funniest bit to me is that I can be completely stuck on a word, I'll stop trying for that word, take a breath and then tell the person I'm talking to that I'm stuck and to give me a minute. Still can't say the original problem word but I can say entire sentences of other words. Stuttering is weird.