Japapanese PSX on USA PS2 with Modbo 4.0

I did some looking around and I couldn't find the answer to this.

So I just installed a modbo 4.0 on my slim PS2 yesterday and it made hardmodding a 3ds seem like a joke.

I'm pretty happy with it but, I can't get Japanese PSX backups to load. Is that normal? Both US PSX and US PS2 backups work fine. The green fix for playing dvd's on RGB works fine. Everything else is fine.

Specifically, I would like to play Metal Slug X. The US version has this white liquid come out of enemies instead of...

Japapanese PSX on USA PS2 with Modbo 4.0