When the printed object is very brittle and there are holes in the object. This means that per traveled distance not enough material is extruded. This underextrusion is caused by the extruder wheel slipping over the extruder wheel. Main causes of this are:

Extruder arm is not properly tensioned. The filament is not pushed sufficiently on the extruder wheel

The extruder wheel is worn. When the teeth of the extruder wheel are not sharp anymore, you can easily hold back the filament while extruding. This should not be the case. It should be extremely hard to stop the filament from extruding by hand.

Bad filament tolerance. Make sure the filament has the proper dimensions. That there are no thickened parts. Or that the filament is too thin. The specified range is 1.75 +/- 0.15mm. 

Bad quality filament. Sometimes filament is not uniform in of material properties. Some parts of the filament require higher temperatures to print properly. When this is needed, try a different filament spool and check if that helps.

Bad thermistor. When the temperature is reading is wrong or offset, then you could print at a wrong temperature. In case that the real temperature is higher than measured temperature, if this offset is too large it could become critical for printing. This results in bad print quality and also the filament could jam in the top part of the hot-end because it starts to become soft too soon. You can check this, by measuring the time it takes to heatup your hot-end from room temperature to 195degC. This should take approx 2 minutes. If this takes too long, you know there is something wrong with the thermistor.

Too fast extrusion. Print slower and possibly with a higher temperatures.