Foxconn motherboard, ram problem

Question

Problem with incresing RAMfoxconn motherboard. 945GZ7MC-RS2H3.06 GHz Pentium 4 Processor.Currently running 2x 512mb DDR2 SDRAM 33MHzI want to increase to,2 x 2GB DDR2 677MHz.The board will run ok with 1x 2GB installed but not with both.I have checked that both board slots & both lots of memmory are working when installed seperately, all seems ok.can you give me any idea what the problem is ?Many thanks.Phil Martin