I notice when I'm nursing my 4 month old DD that there's only one letdown per side (she eats on both each time). I can tell because there's only one spurt of long swallows. I try to keep her on the breast for longer to see if we get another letdown, but she always pops off. She seems satisfied afterwards - she eats every 2-2 1/2 hours when we're together (other than once at night).
I started noticing this because I usually get 2 letdowns when pumping (I work 4 days a week). She's an impatient baby to begin with - could she be missing the second letdown?
I worry about it too because no matter how often we nurse (sometimes 8x/day or more) on the 3 days we're together it doesn't do anything to increase my supply. I thought nursing more often would allow me to pump more during the week, but no such luck.