Multi-disc games not booting

[bevans10037]

I've been able to use PSXPackager to put together single-disc PBPs that load on my PSP Go perfectly, but whenever I try for a multi-disc game that's been generated from an M3U file, the resulting PBP refuses to boot - the console shows the warning screen, fades out, and then hangs on a black screen without showing the PS1 logo or responding to the PS button, forcing a hard reboot.

The game folder is set up the same way as my single-disc PBPs which do work - in a folder with the Disc 1 ID (although I've tried the Disc 2 ID too and that doesn't work either), with the PBP renamed to EBOOT.PBP and the KEYS.BIN file copied in too.