I have this too on the 3drag printer. It can be related to:
- the part leading to the nozzle being wider than the coupler piece before the extruder, so that it requires carefully removing the filament every time.
- the printhead assembly not being tightly screwed leading to gaps where the filament can slightly expand. Sometimes in this case you see leakage.
- air bubbles forming in the filament when heated due to undried storage - this is steam from moisture and causes the filament to expand, making it harder to remove from the printhead. It usually only occurs after many months of storage.