Hi all, thanks to jbernardis for his valuable input. I found the answer after downloading a 3d printer simulator and running the gcode (produced with Kislicer), it became obvious that the missing teeth were due to missing gcode, so after spending time wading through the finer settings of slic3r I created yet another set of gcode and bingo the whole set of gears complete with a full set of teeth printed first time.
thanks to all out there