I've noticed that I can create a field by dragging the mouse from the TOP LEFT corner down to the BOTTOM RIGHT corner; however, I can't create a field by moving the opposite way from BOTTOM RIGHT up to TOP LEFT. (Same applies for all other directions). Currently it seems you can only create fields by moving from TOP LEFT down to BOTTOM RIGHT.
I've tried this in Google Chrome and Mozilla Firefox.
This is a current limitation of the Template Builder software, and can certainly be accounted for in future updates.