Tried playing Metal Slug XX on the PPSSPP core on android, it starts to show the game’s loading screen and then at the point where the PSP system chimes in and tells you this game saves game data, the screen is just black. On the standalone version of PPSSPP for android, this doesn’t happen and the game works as intended. Not sure if this is core specific or Android specific.
Do you have the asset stuff that’s described here (under BIOS): http://wiki.libretro.com/index.php?title=PPSSPP
Most of the “something is missing” errors are fixed by that.