Unresolved Z axis issues - Ongoing

Hi all, I have been having issues with the height of my projects. I recently watched the progress and noticed that every now and then, the robo3D would skip a step in going up the .20mm. The retractor would make the noise, but I noticed that the motor was not turning. This is intermittent but on a project of 100mm it would reduce it to about 80mm. I have absolutely no idea why it's doing that.
I am using 3SD and connecting via USB cable. The X and Y are perfect.
In the photo I have included you can see where it was not going up.
Can anyone please help.
Thanks
George
PS. I am about to try the same thing via USB and then using MC.

 

Is the fan on the bottom of your printer running?