Garageband, soundfonts, sf2 files won't play nice

I have GB3, and am new to it. I was really excited to discover soundfonts, and the possibility of downloading myriad sounds for new instruments, but I have had zero luck with it. I can download the sf2 files, and put them in the correct Banks folder, but when I open them up in GB I can't get them to play correctly:

I can create a new instrument, (e.g. a wooden flute) test it out on the keyboard, fine, and pick it for a track. All's good. But if I stop the playback, and restart, suddenly that new instrument track is silent. If I switch it to another instrument, then click back on "wooden flute," it plays again, but only until I stop. This is true for multiple tracks -- I can have two additional tracks with (e.g.) harpsichord, and all will play fine so long as I've first, after hitting "play," picked an instrument from the original library, then switched to this new soundfont created instrument.

Apparently there once was a program called SoundFont Synth that a lot of people loved, but it no longer exists.

I've tried messing with a demo version of PolyPhontics GB, to see if I could open and re-save the sf2 files (could it help, right?) but that hasn't worked, or at least I haven't figured out how to do it.

What am I doing something wrong? After the soundfont file instruments go silent, I can re-open them, via the generator (DLSMusicService) and start the process over ... regardless of what I do I also can't export the file to iTunes with the new instrument tracks -- just the tracks with original GB instruments play without fault and get exported.

Am hitting my head upon a brick garageband wall. Anyone out there can help?

Many big thanks,
Jennifer