Lol this is an old forum but I second this. For me if I try to start a sentence with certain letters (Cs and Ds being the most difficult, Ws, Is, etc) I get this “block” sensation. There are ways to get around it which is why people don’t notice it very much (I subconsciously mask it). Like I might say “yeah, (can) I get ____” in order to push through the block by putting something before it (like “uhh” or “wait.”) Or my brain just resorts to other letters or vowels that I don’t stutter as much on. But I never stammer in the middle of the sentence, only at the start.