Janome 6600 missing stitches on button hole
 

Hi all, My Janome sewing machine is starting to miss stitches when I engage the buttonhole stitch. The needle is a new one, the thread is suitable &l don't go fast. Since the closest place to service my machine is over 800kms away, any ideas on what is going wrong or what can I do myself? The machine is over 10 years old and was last serviced October 2013, with no problems.
Thank you.