Filament can swell inside the extruder and get stuck.
If you cannot unload filament from one of the extruders, do the following:
1. On the LCD panel, navigate to Utilities > Change Filament > Unload. This option heats the extruders.

2. Set the extruder arm to the open position.

3. Push about half an inch of filament through the extruder to clear the blockage. Try pulling the filament out of the extruder again.

If you are unable to unload filament because it has broken off inside the extruder, you might not be able to reach inside the extruder to clear the blockage. In that case, see the section Disassembling an Extruder.