I had to download the manual for the SM424 to understand what your problem could be. I have to agree that it is likely dull knives.
It's pretty much a baby version of my Bandit
chipper. The nice thing about a full size
chipper with hydraulics is that it actually adds to both the safety aspect and the speed. I would say that a real live full sized
chipper...like at least a 6 inch capacity and better yet a 12...will do the job faster cleaner and in the long run less expensively than a small one.
While you may not be doing entire trees at one shot, the "chuck and duck" type have one major drawback: when the stuff stops feeding it is like working with a stick of dynamite that had the fuse go out. There is no way to reverse the feed. You may be OK trying to jab the stuff in there, but then again....Best to shut it down and clear the jam than risk getting a body part pulled in.
Never ever under any circumstance attempt to force feed a
chipper by pushing into the feed hopper with your hands feet or a metal tool. Use only wooden push stick. Always stand to the side and not behind the feed hopper a a stick can often whip about causing injury.
My bottom line recommendation is to rent a commercial duty
chipper from a bona fide equipment rental firm rather than an abused device from HD. They will even drop it off to you so you don't need to be concerned that you don't have a tow vehicle.
I would put my
chipper up against a fire pit any day. It'll make mincemeat out of just about anything and you don't have to worry about wayward sparks setting the woods on fire. The resulting beneficial mulch is far more usable than ash and there is no burn scar left over. No smoke either.
I admit I have to use a little 5 horse shredder for vines and the bypass leaves and twigs that falls out the bottom of the feed rollers.