I dont know but maybe you need a master card for X1550's as far as i know the only cards that dont are the X1650XT's and X1950's seeing as the X1550 are nothing more then "upgraded" X1300PRO's, another thing HOPEFULLY you didn't pay top dallor for these, got them for cheap if not jesus return them CF anit going to help those weaklings run anything.
"ATI CrossFire technology is an innovative approach to enable multiple video cards to work together in a single system for increased graphics performance and visual quality. Combining two graphics cards in a single system allows you to essentially double your graphics performance. The RADEON X1550 can work in a system by itself or with a second video card. Having a second video card is not necessary in order to use the RADEON X1550, but because the RADEON X1550 is CrossFire ready, that gives you the ability add a second video card later and connect the two together in order to double the graphics performance."
You also might wanna check and see if you have a Crossfire bridge i believe there, TWO bridges, that you must need connected in order for CF to work these can be bought at www.ATI.com/
EDIT: upon looking at the card there seem to be no "connection bridges" i really have lil idea on how the X1550's work but ill keep looking and try to help you