Thank you all for your input. I checked all the suggestions and I think I've probably found the answer. One of the jaws is slightly loose, this is caused by the fact that one of the the machine jaws( the one that the other jaws screw on to) is slightly thinner than the slot it fits into. I don't think that anything can be done about this so I will just have to suffer it and re cut the rim each time I reverse the bowl for hollowing out. I always use dry wood and everything else that was suggested works out fine. Once again this group has come up trumps for me.
Tom