um could have something to do with xa audio, try converting it with a diffrent converter... maybe ZiNgA BuRgA's popstation gui, It always worked for me, and if that doesn't work try a diffrent pops via popsloader...
I'm about to try it with "ZiNgA BuRgA's Popstation GUI" thanks, ill see if it'll work
On v1.2, it didn't originally work for me. Then I did essentially the same thing on v1.3 (except I made sure to hit the preview button to make sure everything was good), and it worked perfectly fine.
When you're creating your .at3 file, are you just using the .wav option with ATRAC3 set as the format? That's the way I do it (and hence, the way it worked for me). Also, did you make sure that it was under 50 seconds and under 490 KB?
I didn't make the .at3 file i downloaded it here. (
http://popsdb.twystneko.com/index.php)