Unanswered Z travel failure

After successfully starting a print and completing several layers the Z carriage appears to stop rising and is just dragged over the initial layers. Is this likely to be a failure of the Z driver or could there be another reason?

 

I bet you need to adjust the potentiometer on you z-axis driver. I had a simular issue and this is the forum post where i figured it out...

http://community.robo3d.com/index.php?threads/z-axis-doesnt-always-work.6392/

Watch the videos at the bottom of the post and use a volt meter...

 

Is the fan on the bottom of your printer running? Usually this is a symptom of overheating stepper driver. If it's on the 2nd layer then your first layer is too low