Problem with Worcester boiler pump?

I have a Worcester 28CDI boiler and i recently replaced a faulty pump. However, the new pump seems to operate intermittently. When i turn the heating or the hot water on, the boiler goes through the sequence and i have tested 240V at the pump terminals but the pump does not always start turning. If i take the cap off the pump and turn it sharply with a screwdriver then it starts turning and runs fine. Any ideas??

Thanks in advance :)