Why does my fan motor blow with less pressure when on heat vs cold?

At first I was getting very little heat from the car. I changed thermostat, radiator cap and added more coolant. Now I get heat, but it is very little pressure even on high. If I simply lower the temperature setting to cold it blows much faster and stronger. When on high heat and vent, it sounds like it should be blowing much more than it is. With most of that noise coming from the dash (even though I have it on vent and nothing is blowing out of dash).