Unscrew the showerhead and turn it on, if you have good flow, there's crud blocking the showerhead.
most of the single handle valves do not have a volume control like on a sink...it's either full on or full off. you only get to adjust the temperature.
Are you saying that there is no hot mixed in at all until you turn it to max?
How old is the water heater? There was a period of time where the dip tube (a plastic pipe designed to direct the incoming cold water towards the bottom) was defective and the eventually break up into little pieces that clog up valves and showerheads. If you have one of those, that may need to be replaced. It would show up as white flecks. If you put some in some vinegar, they don't disolve like lime deposits would.