Walk Behind Lawn Mowers

This is the 2nd summer for my mower, I had always had the setting on a higher setting when mowing, now that the temps are cooler, I lowered my setting to about half way. Now the lawn mower is struggling to pick up the grass. I am leaving a bunch of grass behind and basically having to mow the lawn twice in order to pick up all the grass clippings. Why is this ha happening? I dont want to mow my lawn twice every time out.
Thanks