I am simply having no luck. A second jar is contaminated and the third is not looking so good.
The first two jars went down rather fast so I suspected fungus gnats. I placed squares of yellow sticky paper on top of my two jars, under the screening. Sure enough, fungus gnats.
Next time I will try using an old pair of nylons; my wife's nylons, really. They just spent an hour in the pressure cooker and appear fine.
As for the third jar, the mycelium is still white, but not web-like. The lid on this jar is wrapped tight with aluminum foil and an elastic so I doubt fungus gnats have made their way in.
The stock mycelium is continuing to colonize the cardboard with no sign of contamination and the beech mushroom mycelium is fuzzy as ever with no visible growth into the coffee grounds.