Did you place on freshly downloaded firmware files after? Steps to take when formatting.
- Back up games, saves, homebrew, and any non firmware files somewhere that you can find them.
- Format the MicroSD card using the Panasonic SD Formatter
- Download FRESH firmware files, plug ins, etc. Anything related to the firmware or extra functions of it that things such as plug ins provide
- Place these fresh files on your MicroSD card correctly
- Put all games, saves, homebrew, etc. back onto your MicroSD card
If that doesn't work, try a different MicroSD card.