ive just bought a 08 yamaha ybr125, ive had it for a week and only done 300 miles on it? however when im stopping and going down the gears it seems to be missing them? ie when i pulled up outside my house and went from 3rd to 1st then neutral it just wouldnt go in to it, and went straight to 4th then wouldnt go back down......some help would be very greatful as this is my first bike and im not to sure