A few things that you need to check, to cancel out things that are NOT causing the problem, thus allowing yourself to figure out IS actually causing it:
1.) Open GAME.txt and make sure the SPELLING on ms0:/seplugins/infmic.prx 1 and ms0:/seplugins/pspbricker.prx 1 is all correct. That is the number one issue when a PRX won't work, is spelling.
2.) Check in Recovery to make sure that GAME plugins are enabled.
3.) Check to see that PSPBricker.txt inside of SEPLUGINS is NO LARGER than 8kb. That is the maximum size for that file, and if it is larger, it will cause PSPBricker to have all sorts of errors.
4.) The PRXs may have been corrupted, along with alot of other things during downgrading. Backup everything inside PSP/SAVEDATA on your PC, format your memory stick, and install everything again, from scratch. Then put the backed up SAVEDATA folder on your PC back onto your PSP
5.) Check your Clock Speed in Game, if it is set at Default, try it at 333/166, and it may work for you, no problems
6.) If you are having problems with NitePR, make sure none of your text files (codes) are larger that 50KB, because that will F***K up NitePR sometimes.
Hope that somehow helps