Mac Pro not recognizing extra RAM

hi gang,

i just got another 2gigs of ram for my 2.66 mac pro. bought it with the stock 1gb, added a matched pair (2x1gb) and now just added another matched pair (2X1gb). powered up and the computer only recognized the 1gig sticks (all four) but not the orig 2x512. i have placed the ram in the proper slots and made sure they were seated properly but About This Computer has DIMM riser A/DIMM 3 & 4 listed as empty. that is where my 2x512 sticks are. any ideas?

(the newest ram is being recognized, so that's no prob. my mac just "forgot" about the orig ram i guess)

thanks