If the thermostat is stuck closed it won't flow, , and it also won't open until the motor is hot.
If it was mine I would pull the thermostat, test it in a pot of boiling water , or over a candle to make sure it opens when hot or just replace it without testing it. While you have it out do some flushing to make sure there isn't any restrictions from weeds,sand,or old impeller pieces.
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