There is a few reason that can cause this and LambdaFF touched on one of the most common ones. In the past I have had this problem but adding a new script into your printer settings seemed to help me.

Printer settings --> Script --> Script 1 (rename to something like "Run On Pause" --> Insert below Gcode --> Apply
G91 ; set relative positioning
G1 E-2 F1800 ; retract filament slightly to avoid drips
G1 Z20 ; move extruder up 10mm
G90 ; return to absolute positioning

Hope this helped.