Maybe it's the yeast?

[NeoCarpetBagger]

Okay, this is just weird.
I'm a pretty experienced beer brewer, just starting out in distilling. Just saying that I've been around the block a time or two, but using new tools.
Anyway: I start a mash with the "Cornflakes" recipe. 1 box of generic cornflakes, 3.5 lbs of piloncillo sugar, 4 lb of white sugar, 5 tsp of Yeast Nutrient, and 1 tsp of Amylase enzyme (just to make sure of the starches in the corn.)
I get a nice SG of 1.064 (yeah, probably a bit low.)
When the mix cools below 100 F, I pull out a sample, place it in a smallish jar, let it cool a bit more, and add 1/4 Cup of Dry Active Distillers Yeast. Within 10 min I have serious activity in the jar. This yeast is Happy! The main batch gets below 90 F, and I pitch and seal the mash. Pretty standard so far, right?
I place the ferm-lock and wait. And wait. And wait. No bubbles in the lock. WTF? Did I somehow kill my yeasties? Well, fermentation can take a bit, so I leave it alone for a few days. No bubbles. Not one.
I give it a week. No bubbles. I'm seriously concerned at this point, so I take a sample to find out what's going on. The hygrometer reads 0.998. Fully dry. WTF?
I run it through my Chinese no-name pot still, discard the first four ounces or so, and the product comes out at 55% and is seriously delicious.
Okay, the fermenter is a 6 gallon bucket, and I was using a "gamma lid" (press-fit ring with a big screw-in insert) maybe I had a bad seal, and the exhaust gas had a different route out. It happens.
So I try the recipe again. The only change is that I use a full, press-fit lid on the fermenter. After placing the lock, I press on the center of the lid to make sure of the seal. Lock lifts. It's tight.
Again: Not. One. Bubble. Again, FG 0.998.
What the actual h*ll is going on here? Obviously something is, I'm getting fermentation, just none of the expected exhaust. Huh? Shouldn't be possible, should it?