my 4800 does that too. It seems to like the outer half of the bobbin thread but not the inner half. If you switched out your empty bobbin with another full one, it might have been the bobbin was not the same size or maker. Janome's are finicky about bobbins. Take the throat plate off, take the bobbin race out and clean well for any lint. Mine gets cranky if there is even a tiny piece of lint down in there.