This occurs due to multiple causes.

(a) Nozzle could be blocked or partially blocked

Solution: Not enough material is released from nozzle

(b) Too much pressure is put on the material by the feeder

Solution: Release the pressure on the feeder a bit by adjusting the build plate screw clockwise

(c) Too little pressure is put on the material by the feeder, causing the material to slip

Solution: Put more pressure on the feeder a bit by adjusting the pressure crew counter clockwise.

Posted in: Ultimaker