I had a similar problem with my bike. I was holding the shifter lever too long. I thought I had to hold it until I heard it shift. But I have now found that I just need to give the lever one little movement and let go...shifts now smooth as butter. If you don't think that is your problem, I suggest taking it to a shop who will put the bike on a trainer so you can ride and shift gears while they check out what is happening. Good Luck!