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Originally Posted by Truckjohn

Some tips:
1. Sharpen that plane and take finer shavings. Your shavings are still very thick for good jointing. For jointing, you want nearly transparent shavings.
2. Close down the mouth to reduce tearout.

Technique:
General comments, not in order.

Planes cut heavier where you're clamping/supporting the wood and lighter where you're not. Yes, even on a 9" deep section. I generally start with support around 1/4 of the way in from each end and see how that goes. YES it makes a difference. From your hollows, I would guess your clamps are toward the ends. Move them in toward the center some and take a few passes. You absolutely can compensate the plane cut by shifting your support location. In fact, that's how I do it.

Work to get rid of your humps with short, carefully aimed strokes directly on the high spots. The plane must also be set extremely fine. THEN, when the only defect all the way down is the little lump from the start of a shaving, take 1-good full length shaving. Continuing to take full length shavings when the geometry is out of whack only disrupts your geometry more.

Don't tape both halves together when jointing. Do them one at a time and candle against a quality straight edge. Doing them taped together only ends up producing wonky geometry on both plates at the same time. Once both pieces candle to a quality straight edge, candle them against each other and adjust ONE half to match the other. Don't chase the joint on both halves at the same time.

Need practice wood? Paint stirrers are free or extremely cheap. Jointing them is fantastic practice, and it won't wreck EXPENSIVE tonewood.

Early on, I did two "Tops" out of 1" wide, 1/8" thick spruce slats. I think one "top" came out 16 pieces and the other 15. After that, I had my technique down, and jointing actual top and back sets was a snap.

Keep at it. You'll get it.

The block plane might be the problem.