I haven't tried another soundfont player, but I have found one other soundfont that didn't work (since deleted) so it might not be something with the soundfonts themselves.
I just bought the full version of FL and the soundfont player a few days ago, so I'm up to date. I've downloaded about 150+ soundfonts and other than your batch, only 1 hasn't worked so far. It'd be nice if I could get them to work as I haven't found many good free Piano ones.
The process I use is the same for all my soundpoints; I unpack it if it is a sfArk or something like that and then add it to a folder within FL that shows up on the browser. I then drag+drop them to the pattern.
I'll keep tinkering and see if I can find out what it is.
EDIT:
I just deleted them and re-downloaded them, restarted FL, and they now work. Weirdest thing. I'm not familiar enough with FL to know what could have caused it; the original downloads may have been corrupted in some way but I'm not sure. Either way they're working, so I don't care.
Speaking of soundfonts, this has been a good resource:
http://soundfonts.homemusician.net/
Not as good as ones you'd pay for, but if you're just getting started with soundfonts and want something that doesn't sound like total garbage it's a good place to look.