This is like removing a screw top lid stuck on a jar. You can try lubricating the filter cap with a little water or cooking oil to make it easier to turn. Or, you can buy a sheet of rubber like material that is intended to help you grip the lids of jars to make it easier to twist the lids – try using one of those. You also can try running hot water over the filter cap (not the chamber). The filter cap expanding with heat may make it easier to turn.