It might be the card itself as I am using a 16GB card in my device and it is working just fine. Mine is a Sandisk 16GB card I got from Amazon.
A few things you might want to try.
1) make sure the card is formatted properly. Using windows or MACs formatting functionality does not always format larger cards correctly. if you have a Windows machine, use this formatter which formats to the spec according to the SD card association.
http://panasonic.jp/support/global/c...formatter.html
Make sure you check in the options are set to Format Size Adjustment "ON". (It is off by default) When this is set to off, some devices have problems with larger cards.
2) Make sure the card is mounted when inserted. You can see that in the settings under SD Card & Storage.
Hope this helps solve your problem.