Fujitsu Stylistic LT C-500 needs memory

I have a Fujitsu Tablet that came with 256mb RAM and I installed 512mb RAM. Computer runs fine, but does not recognize more than 256mb RAM. It is running XP Pro Tablet Edition SP2 - it does not recognize the extra RAM either. The bios has no facility to change the amount of RAM. Any suggestions on how to get the extra memory recognized would be greatly appreciated!