This sounds like user error. It is likely that your extruder gear is spinning as intended but the filament itself has had a groove ground into it by the gear.
This can be caused by a number of reasons as follows:
-
Printing at too low a temperature, if you do this there will be excess pressure in your hot end which wont allow the filament to be extruded at the proper rate. Your extruder gear then over time grinds a notch in the filament and no longer can grip it properly resulting in mid air printing with no extrusion.
-
Too much tension on your filament feed, where are you housing your filament spool? If there is too much tension on the line the filament wont feed properly and you will also result in a notch ground into it same as above. Generally people hang filament above the extruder and if possible do so on a holder that has a bearing ensuring minimal resistance.
-
Excessive retractions, if you retract too quickly or for too long a distance, over time you also have the chance of grinding a notch in the filament. If you are printing in PLA try a retraction distance of between 1-3mm at a rate of between 20-35mm/s
-
Finally, printing with too low a starting Z-offset. This is less likely to be the issue as you say that the failure occurs multiple hours into a print