Hi,
You may want to try reformatting the cards. Backup the data on the card (like to a PC using a card reader) and then reformat the card on the phone. For information on how to reformat a SD card in the phone, please refer to the Using an SD card to add space page in the online How-To guide
After reformatting the card, copy the backed up files back to the card. If reformatted the card doesn't solve the problem, you may need a new card. The flash memory used in SD cards will wear out. It can be read many, many time, but can be written to far fewer.