Rear derailleur's range too small?
Hi, I've been building up an old CAAD4 frame I bought on the cheap and found a problem with the rear mech that I just can't fathom. The bike came with a Shimano RD-5500 9 speed derailleur that I've kept, but I've had to fit new wheels and a new cassette. The cassette's a Shimano HG50-9, so it all should work together but the derailleur doesn't travel far enough to engage either the bottom or top cogs. I've let the H/L screws as far out as they go but no luck there - it only chatters slightly when trying to move into bottom gear and doesn't come close to moving into top gear. I've also disengaged the gear cable and moved the mech by hand - it just doesn't seem to move far enough in either direction to cover the 9-speed range.
Originally I thought the hanger could be bent, but it doesn't look to be and I figured that would only effect one extreme, not both.
So does anybody have any ideas? Or have I just made a massive/embarrassing compatibility error?
Thanks for any help