Dictation may stop working midway through a sentence if you pause for too long while speaking. The dictation feature may interpret a prolonged pause as the end of your speech and automatically stop listening. This can happen even if you have not finished dictating your message.
To avoid this, try to speak continuously and maintain a natural flow while dictating. If you need to pause to think, be aware that a long pause may cause dictation to stop, requiring you to tap the microphone button again to resume.
However, a long pause is only one possible explanation. Dictation may also stop because of a temporary a network issue.