It happened to me with different manufacturer. It never cured and made such a bad mess I phoned and flamed the manufacturer (GE), they replaced the product but had to toss the boards.
The Dow 738 product has a 7 month shelf life, I would check the date/batch. It could have been sitting on a shelf for a long time or maybe got frozen.
It still relies on moisture in the air to cure, but low RH has never been the reason for troubles.
So far, it's old silicone that doesn't harden, with the new formulas.