Bit of a noob, tried searching around but not sure if i could find anything useful/meaningful to me

Running 6.20 TN-D HEN on an "unhackable" PSP 3000
Using iso loader, I am able to play newish games fine eg god eater burst

However, I am unable to load older games. Main example being metal gear acid 1
It does the PSP logo thing, and then just hangs on a black screen, then after a while PSP turns off.

Is there a compatibility issue here?
I have tried patching the ISO with Iso loader prometheus patch but still no joy.

Thanks for any suggestions