tldr; a stutter's brain is wired very differently because of our weak motor-speech centre, as a result we have timing problems in producing speech because while we're speaking the brain cant accurately and efficiently correct speech errors(like saying the wrong word) the same way a non-stutterer does. Many studies with fluent speakers as a control group have proved this. And speech therapy helps because the brain uses the right hemisphere to overcompensate for the weak speech motor problems in the left hemisphere increasing fluency and reducing the cognitive difficulty a stutterer experiences while speaking(such as formulating thoughts, words and the added fear accompanied with stuttering). The read was very technical with mentions of specific neurons and many studies, but that was the overall message. At then end of the day, the root causes of stuttering(etiological the technical term) is not fixed and thats why you might block even if you're fluent.