Walk behind Lawmboy model-10647 Grass Catching/Bagging problems

Hello, New member here. We have a recently purchased Lawnboy walk-behind, rear-discharge model-10647. This is a rear wheel drive, electric-start model. Everything is great about the mower, except that it will not fill up the bag with grass clippings. It seems to clog right at the beginning of the bag with about 3 handfuls of clippings. Damp or dry grass does the same. I've tried replacing the multching blade with a standard blade....no help. I've tried mowing in about 8" paths...no difference. Tried walking/ mowing very slow. Makes no difference.
Is there anyone else having this type of problem? I heard somewhere that this is primarily a multching mower, and that nothing can be done. The Discharge opening is only 3" x 7". Very small. Any possible modifications that someone may have come up with? Thank you!

This is a 20" mower. Not sure if the machine has a throttle control (probably not). If your engine speed is below 3,000, you might not be getting enough blade speed (airflow). For a 20" mower, your high speed limit should be around 3,400 to 3,600 for MAXIMUM performance. For safety sake, governor lag and a little fuel savings, I shoot for 3,200 to 3,400 on the 4 cycles with that blade length.

Thank you for your quick reply! Yea, there is not a manual throttle control on it. But, you have me thinking! The problem Does seem to look like there is not enough airflow to push the clippings in far enough! This mower is a Calif model. So, it's probably full of Emission controls. I will see if I can bump the RPM up a little. Many Thanks!