I have an issue with my rear derailer on my MTB, it's a Shimano 6 speed, and it's always a gear in front. i.e. when I shift to 3rd gear, it jumps to the 4th sprocket, etc. When in 1st gear, it's on the 2nd sprocket, so if I adjust the chain and manually place it on the 1st large sprocket (i.e. 1st gear), and set off, it immediately jumps to the 2nd sprocket, giving me 2nd gear, then I'm back to being a gear in front. So, I'm missing a gear, as I have no 1st, and I'm always a gear too high. I haven't 'messed' with it much yet, thought I'd ask first, as it may be a simple adjustment which I can hopefully do perhaps. Thanks!