Filament lifts from the hot bed while printing
Check the following:


Is the print bed clean? On glass, you can use a few squirts of window cleaner.
Is the print bed actually reaching the correct temperature? 
Have you manually calibrated the printer (at both the center and the edges), such that you can just about get a sheet of paper between the print bed and the hotend nozzle, at z = 0? This last check ensures that the first printed layer of extruded filament is actually touching and "presses on" to the print bed. See the video #18:Calibration for a great explanation on the use of the paper. Whilst this video is for a Delta printer, it clearly demonstrates the height that the zeroed print head should be at, and how to check using a sheet paper.
Maybe the filament, for the first few layers, should be heated at a high temperature, than the rest of the print, to ensure adhesion.


If all of the above are checked and OK, then (as electrophile points out in their answer) try making the print head more grippy. This can be achieved by simply adding a thin coating to the glass with one of the following:


Wiping the glass with a glue stick or wood glue
Using hairspray


Both use PVA as an adhesive/stiffener.

Or by adding an additional print surface, such as:


Using blue painters tape
Using PEI tape
Using Kapton tape
Using BuildTak


If adding tape, then the printer may require a small re-calibration, due to the thickness of the tape adding a few microns to the print bed height. This can be done in the firmware.