Possible causes of print defects?
There are many possible causes for printing defects, but the easiest one to eliminate is dirt. Always use a filament cleaner to prevent dirt from entering the extruder. There are plenty of designs available on Thingiverse. Always keep your nozzle clean. After pre-heating mine, I clean it (carefully) with a folded paper kitchen towel. It may also be worthwhile doing a "cold pull", if you suspect that you have dirt in your nozzle. Finally, you should check that filament is not leaking out between the nozzle and the heater block.