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Hello, there's more than one possible cause of this. For example, although the pump spindle is rotating there could be a blocked or loose impeller. There could be a blockage elsewhere in the system.How old is your boiler and the rest of the system?
Did you dismantle the pump, i.e. remove the head from the body so that you could see the impeller?
If you're feeling adventurous, you could uncouple the CH flow pipe from the boiler, assuming that the filling loop is connected to the CH return, close down the bypass valve, close the valve on the return into the boiler, couple up a temporary pipe from the CH flow into a bucket, and open the filling loop. See what comes out under the "encouragement" of mains cold water. Be careful not to overpressurise the system though or you could encourage leaks.