Fresh Lemons, who made some darling bags as part of the wrap-along. She suggested I spritz the gathers and go at them with a hot iron and plenty of steam. I did, but even though that was a great suggestion and proved helpful later on in the process, it wasn't the root of my problem.
After posting my pitiful problem bag on Instagram last night, Jana_Nielson and Busybean came to my rescue. Between their suggestions and a little web searching, I figured out the solution to my issue. See below, my two test samples? The one on the left didn't gather at all, where the one on the right looks great.
this post by Simple Analogy that gave me the real clue: check out the second and third picture in her post, where it says 'WRONG' and 'RIGHT'. Seriously - hop on over and check it out while I wait. You need to know this. la la la la la . . .
Ok, see? My elastic thread in the bobbin was not catching in the tension guide!
So I manually pulled it up with the flywheel, but even then, it was a bit tricky. I found that if I held a little tight to both my upper and lower threads, it would catch - something I could easily see if it was right or wrong, once I knew what to look for. And it made all the difference!!
Just for the record, it still took the steam Faith suggested to make it gather, but gather it did. It's darling just as it is, but I though I'd also show you what a contrasting ribbon can do.
Whew! All that for one little bag, but now I know! And now another of these bags will come together in just a few short minutes!