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