at what height are you cutting?
the higher you cut, the farther the deck is from the surface and the less complete suction there is.
Also your RPM/engine speed could be suffering for some reason.
What height/how lush is the grass?
Typically you want to be cutting 1/3 the height of the grass, then your suction is good.... if you are just hovering over 1.5" high dormant lawn, expecting to suck up everything.... you might be disappointed.
In my experience a faster pace and a double cut is a better strategy.