I have a Vertical BMX bike. My problem started small. Usually when I started from a stop or put a lot of pressure on the pedals going uphill, my chain skipped or something to that effect. Now it gets to the point where I barely ever get anywhere pedaling. What's happening? I've tried moving the back wheel further back, but that doesn't seem to do a whole lot. Any other suggestions as to what the problem might be and how I fix it?