PierreFablab

I possess since december 2016 an ultimaker original+. It works really great, but I have got a little problem on most of my prints. It is a layer problem on Z axis. The first centimeter of print is really smooth and regular, but around 1cm on the a axis, some layers are missing, at three places. And there is no problem on the higher layers. The place of the problem is always the same. What can I do ? Do you think that the two ball sleeve are faulty ? Thanks a lot