My reliable NSM Galaxy 200 jukebox is suddenly refusing to play "A" sides! What is likely to be the problem?

The playing arm rests the needle on "B" side selections and plays them OK, but when A-side selections are made, as soon as the needle on that side of the arm touches the record, it rejects. Both needles appear OK. I recently moved the jukebox a few feet while decorating, but it did not get any jolts that could have upset the mechanism... and it travelled safely across the width of Europe six months ago without this problem appearing. Any advice appreciated. John.