I've switched out my Shimano 105 10 speed cassette and put in Ultegra 10 speed cassette. Before the switche the chain ran perfectly up to the top cog and down to the bottome cog. Both are 12/25. Now I can not run the chain to the top (easy) cog on my rear cassette. I've backed off my low gear adjuster about 1.5 turns and it still will not run up....any ideas? I don't want to back off the adjuster too much in fear of losing the screw from being too loose, or running my chain off the cog into the spokes. I apprecitate any feedback.
Did you put the 10-speed-specific 1 mm spacer back in place when you put the new cassette on the freehub body? If so, then I'd say the cassettes are just a bit different in terms of where they sit laterally along the hub's axis and you should retune the RD according to Park Tool's directions: http://www.parktool.com/repair/readhowto.asp?id=64