Pumping output?

I’m not sure if I’m just not seasoned enough to figure out the right settings on my pump or if this is normal. I pump usually about 2.5-3.5 oz each pump depending on the time of day but I get all that within the first 4-6 minutes. I try going back to massage and tweaking the settings to get another letdown with no luck. I have tried pumping for longer than 20 minutes as well as power pumping with the same output. Is this normal? Or does anyone have suggestions for settings that have helped them with multiple letdowns?