People delay switching for two reasons: fear of giving offence, and the belief that discomfort is "part of the process".
The second is worth splitting in two, because the distinction is real. Discomfort that is part of the work: you feel exposed, a session touched something painful, you want to avoid the subject. That is a sign you're working, and avoiding it is exactly what brought you there. Discomfort that signals a mismatch: you don't feel heard, the therapist talks more than you do, gives advice you didn't ask for, challenges your values, or you simply feel nothing after eight to ten sessions.
The practical test: tell them. A good therapist receives feedback as working material rather than as an attack — and how they react gives you the answer. If they become defensive, you have learned what you needed.
You don't need an approved reason. "I think I need someone else" is enough. And a professional will help you find that someone — which is, in itself, the sign you had picked well, even as you leave.
One exception worth naming: if you change therapist every time the work gets hard, that pattern is worth discussing before the next change.