That was my first thought. A jointer depends on the number of cuts per inch to develop a flat/smooth surface. If you push the board through faster, you develop scallops. Fewer knives means more scallops. Some jointers have only one knife and if you go very slowly you get a pretty good surface. The RPM's of the head have a similar effect.
Use a steel rule to check the blades. Mark the knives with a magic marker so you can identify them. With the machine unplugged, set the rule on edge on the out feed table and turn the head toward the operator end. Note how far the rule is dragged. Do this on both ends of each knife. With the out feed table properly adjusted or the knives set correctly, the rule should move the small amount each time. I find mine works best when the knife just snicks the rule and moves it about an inch.
Face jointing a knotty snarly piece can be something of a rough ride anyway. Sharp knives help. Firm pressure with push blocks helps also.