While using your Ultimaker it is necessary to periodically calibrate the build plate for good adhesion of the print to the build plate. If the distance between the nozzle and build plate is too big, the print will not stick properly to the glass plate. On the other hand, if the nozzle is too close to the build plate, it can prevent the material extruding from the nozzle.
To start the leveling procedure, navigate Maintenance > Build plate in the menu.
Take a look at the Get started section for detailed instructions on how to level the build plate.