Many sounds seem to give me trouble to varying degrees, but "F"s at the beginning of sentences are especially tough. Might be relevant to the question-- there's a video of Samuel L Jackson accepting an award talking about his stutter that stuck with me. He says his stutter seemed to change all the time. "Some days were 'B' days; some days were 'S' days..." or something to that effect. I'm on mobile, or else I'd link for you.