Need help with CPS2

Hi guys I'm having a little problem and I'm hoping somebody can help. I had CPS2 working perfectly on my memory stick. The problem is i change the bg and then it didn't work. Now i have downloaded all of NJ,s New emulators and none of the CPS2 seem to work for it. It tells me it needs to be a 2000 psp or it gives me the error cant load massage. But the CPS1 2.3.1 slim works fin on both my path and slim PSP. Any help on what it can be. I know it was a newer version because i had to reformat the games. and it worked find but now it doesn't. :(