Irene, the only thing that I can see that may be causing this is something that was in the threads of your unit when it was closed last. There is no way for any debris to get to the plug from inside.
Try turning it back and forth a few times. If that will not loosen it then you will have to apply more force to it to get it out. Iknow this doesnt sound like a good solution, but unfortunartely it is the only one there is. These plugs simply cant get anything in them from inside the unit. As there is no way to get the cap out otherwise Im afraid this is the only option.
Please let me know if you have any other concerns.
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