For a long while I was having this problem mainly between 4 & 5 gear as well as downshifting to maintain hill speed. It was also hard to find a gear again between downshifting and up shifting again to find it. I thought I may have had a manufacturing problem. It turns out, my problem was all technique. Through my riding program I was taught to use a firm foot and fully lift or press the shifter to it's limit. However, after some fatherly advice on my recent trip, I started shifting with more finesse and a lighter flick of the toe. I haven't had a problem since.