I just changed my cassette for the first time. I went from a 12-25 to a 13-30, both shimano ultegra 9 speeds, and everything seemed to go smoothly. But when I tried to run through the gears, I could start at the smallest cog and move to the 2nd, 3rd, 4th and 5th, but it would not shift up to the 6th cog. It tries, it goes up on the cog, but won't stay. What could be causing this? The spacing between each cog looks to be the same, this is a 2 tooth jump - 19 to 21 - but the shift before it is 17 - 19 and it has no problem with that, so I don't have a clue what the problem is - - - ideas?