Piercing Time is the time at which Mr Beam II burns into the material at the beginning of each new line or shape without moving the laser head. 

If cut or engraved shapes are not completely processed on certain materials, such as e.g. not completely closed circles or rectangles, then you should set a pierce time.

Try to set a pierce time for your laser job:
In the Parameter assignment for a laser job, enable "show advanced settings" (at the very bottom):  

Enter a value for pierce time for the cutting and/or engraving job. 100ms is a good starting value. This is the time which Mr Beam pierces into the material without any movement at the beginning of every new line or shape before it starts moving the laser. 

For some materials Mr Beam II needs more energy to do the first pierce of a cut or engraving than it needs for the ongoing cut/engraving. 

You will find more in-depth information about laser parameter in this article.