I hate these type of bloody inconveniences - i've got the same on some campag running gear and all I did was change the cable - it shifts but 9&10 are the same gear - one press on the ergo and 8th to 10th just like that - but I wanted 9th you ****** thing!
anyway - to your obstinate gearing - how's your chain? If that's worn and you've changed the cassette you could have a few issues with crisp shifting? Or the jockey wheels themselves - if they're worn that again won't be helping the job.
6400 is 7-speed, 6401 is 8 speed & says integrated 8 SIS on the jockey wheel cage.
Rick Powell 753 Track / Ribble Gran Fondo / Colnago Asso / 1975 Harry Quinn TT / 1990 Concorde Prelude / Vitus 992 / Keith Coppell Lo-Pro / 1986 KAS Vitus 979 / 1995 Mercian Strada Speciale / 2011 Raleigh GAC Militis 3 ex Graham Briggs / 1995 Ribble 7005 / 1986 Alan Super Record / 1985 MK1 Vitus 979 /
On the bench: Sirius 853 TT / Sun Solo Chrome / Ciocc Designer 84 / Giant TCR0 Pro Compact / Ribble 653 Tourer / Daccordi Lo-Pro / Look KG96 / Bottecchia Equipe / Look KG231 Credit Agricole / Contini Grand Prix / Aende Lo-Pro / TVT / Langdale 653 Pursuit /