Good point, Leon. I know about the wind in Kansas. It blows potato chips off the plate while trying to eat outdoors.
I've heard rumors that some machines are picky with thread, but not a 201. I'm guessing you are using a spool pin felt under the spool of thread, so that it can feed freely. I'm wondering if you have cleaned out the tension assembly and oiled after sewing all those masks. Another thought is that somehow the check/take-up spring somehow got out of position or that accidentally, when threading that it didn't get in position over the retaining fork. In my 201 manual the retaining fork is number 3 in the instructions for upper threading.
I'm not sure it would make any difference, but have you checked the needle to make sure it isn't bent or blunted and the correct size for the thread.
Janey - Neat people never make the exciting discoveries I do.