I would also suggest adjusting the actual gear selector (the bit your foot moves). I found that I occasionally missed the 1st to 2nd change as my foot didn't come up quite high enough, I've adjusted this so that the selector is now further down and I don't remember the last time I missed the change...
Had one issue of the bike refusing to go down, but that was from 2nd to 1st and particularly high revs and I didn't realise I was in 2nd :P Bit of a muppet moment haha