Two things I can think of: there should be a fill line on the tank - does the water fill to that point; if not, then adjust the fill device. Second, some flapper valves are specially designed to stay open to allow the specified amount of water to be used - you may need to replace the flapper valve with the manufacturer's rather than a generic. The float may let it stay open longer than a generic one.
Last, you can try to hold the handle to let (most of) the water flow out for a flush. If it flushes well, and the level is up to the fill line, then the flapper is dropping too soon. Otherwise, you probably have an obstruction.