Timing---are you sure it isn't a "summer hols" phenom?
Tech--i'm one of the posters who don't have and are not going to acquire any time soon, the equipment set-up and technical know-how to post. but i've gotten a big kick out of listening to the submissions and messing with the selections that i liked.
Taste---RE "the selections that I liked," truth is, it's not that many. i think the OP hit on a big factor in the possibly low participation....it's the bind of, while it's totally fair and understandable that tastes vary widely and not everybody is going to like all the selections....folks are usually only going to bother with pieces they really like. so that's a dilemma.
Taste, Two---i'm also a melody player like the poster earlier in this thread, who said they like concertina for itm and other melody muslc. over time, i've come to understand that many concertina players particularly with duet and even the ec, are into it for its chordal and/or counterpoint capabilities. i really enjoy all the different sound posts, but at the end of the day., i'm not gonna bother much with that stuff. come to think of it....i guess, if participation were more robust, one thought would be, forums re different interests. a "classical" emphasis, or a "world folk genres" focus, etc.
Training: I personally would not use an activity such as this in the context of an internet forum, to get "feedback" on playing technique, or teaching. i want "feedback" from documented master artists and/or master teachers. with everybody else i just want to have fun sharing. on the other end, i would never offer it on an internet forum unless there was a specific request stating it was wanted. and even then.....True, true, Concertina is such a specialized instrument that lots of beginners don't have teachers accessible, and they really need help. But seeking it on an internet forum should be done with a big ole caveat emptor. [For Anglo at least, there are fantastic people now giving Skype lessons.] If splitting into different forums or threads isn't something folks find appealing, perhaps "feedback" could become optional, with the beginners or folks who want "feedback" free to solicit such, and again, caveat emptor.
Titillating Thought: What about a shift to making it, "MY" TOTM? Like, "Monthly Tune Show-and-Tell?" A deal where everybody just shared a tune THEY had selected and worked on to share for that month? People could post about the backstory for the tune, where they sourced it, why it drew them, etc., along with their sound clip. And then the next month, everybody would pick another one and share-and-tell about that one...... 
Edited by ceemonster, 23 September 2014 - 08:57 PM.