Layer separation

If the layers does not bond / melt together and individual layers warp within your part, this is called layer separation.

The adhesion between layers can be increased with higher printing temperature. Although if you recognize that the extruder sometimes has a hard time to push the Filament trough the hotend, it´s maybe because your nozzle is to cold.
Try to rise the temperature by 5-10 degrees so that the plastic flows easier.

Look at the pictures below to find out how to do this.