lol you dont need 1.5 to make pandora's battery with the firmware you have now 3.40oe-a you should be fine so try making that but try these options to
go to recovery mode and then to configuration and make sure these are like mine (if your on 3.52 that is if not just check the ones that apply to you)
1. skip sony logo (currently: Disabled)
2. Hide corrupt icons (currently: Enabled)
3. Game folder homebrew (currently:1.50 kernel (try changing it to 3.52 see and see if it works)
4. Autorun program at /PSP/GAME/BOOT/EBOOT.PBP (currently: Disabled)
5. UMD mode: (currently: Normal -UMD- required-)
6. Fake region (currently: Disabled)
7. Free UMD region (currently: Disabled)
8. Hard Reset on homebrew (currently: Disabled)
9. Use VShMenu (currently: Enabled)
XMB Usb Device (currently: Memory Stick)
some of these have nothing to do with ISO's loading but just to be on the safe side i put them up
also are the files .iso's or cso's if they are iso's try converting them to cso's with UMDGEN4.0 you can get
here and also make sure you have a UMD GAME disk inside
if nothing works if may be something internal