Campag and Shimano use different spines on the freehub. Got a photo? The wisdom of the internet also says 8 speed Campag hubs are different than 9 speed.
All of my hubs are Campag 9 speed, with the earliest being a 1999 Record - I've never had a problem with cassettes, I've 3 (or possibly 4) Campag that I've bought over the years and a recent Miche in Campag fitting.
All fit with no problem so I'd suspect that you've got a Shimano pattern cassette or that you're not aligning the index spline when trying to fit it.