Underextrusion (as this looks like) could be a extruder problem. The extruders main part is the small gear that pushes the filament through the hotend. There could be lots of reasons for it to fail:

• The motor driving it isn't strong enough. Poorly designed printer (yeah, ultimaker, I'm looking at you). Not very likely. But a geared extruder or motor can improve extrusion.


• You print at to low hotend temp. If you increase the temperature the filament will become more fluid and easier to push out. Increasing the temperature will unfortunately increase risk for clogging and stringing.


• The filament has absorbed moisture. Try drying it.


• You print to fast. Lowering the speed may do the trick. You also have to wait longer (dhu).


• The acutal gear in the extruder is full of filament bits, clean it.


• You don't put enough force on the filament against the gear. Can you tighten it?


• The gear is not hobbed. A hobbed gear will grip the filament better. You might not be able to switch it.


• The filament has become tangled on the spool. Be careful when you handle it to not unroll it. This usually results in that the filament becomes entirely stuck though.


• The nozzle is clogged. See answer from @Tom van der Zanden.

If you don't have a geared extruder you can print one. There are plenty of them on thingiverse. Increase the temp and lower the speed to get this vital part in good quality.

Or you can buy a geared extruder. (I use the titan extruder from e3d)
Or you can buy a geared motor.

How to detect an extrusion problem:

It is quite easy to hear if you have extrusion problems (at least some of them). The Extruder will make a sound like "drrrr" with regular intervalls as it tries to push through the filament. Almost like a clock ticking but slower. This is the gear slipping against the filament.

If you don't have an extrusion problem:
Experiment with upping the Material flow rate in your slicer (in your case Cura). Try something like 300%. Stand watch and see what happens.

If this almost fixes the problem, you must calibrate the printers extrusion. It is possible that the already generated Gcode (found on the sd-card) compensates for this.