If your glass build plate has been handled a lot or is dirty, you should remove it from the printer and clean it with warm water and mild soap. Touching the top surface of the bed with your fingers will leave oils on the build plate and prevent filament from sticking to the plate.
The most common cause for prints not sticking to the print bed is that the bed has not been prepped for printing.
Two popular choices for prepping the glass plate include using Elmer's disappearing purple glue stick or Aqua Net Extra Super Hold hair spray.
If you use the glue stick, you should apply a thin layer to the glass plate at room temperature.
If you use the hair spray you can apply to the bed at any temperature, just be sure to allow it a couple minutes to dry.
If you are still having issues getting filament to stick, you should check the calibration of Z Height of the machine. When you perform the Z height calibration with the LCD controller, you should bring the nozzle down all the way to the bed to the point that it is beginning to pinch a sheet of standard computer paper. This will leave the nozzle approximately .004" above the glass plate.