A recipe I'm doing calls for making something close to buttermilk but less tangy by mixing Greek yogurt with whole milk and letting it sit out to ferment for three hours to overnight until it bubbles. It's been five hours; no bubbles, still tastes like yogurt. I used Greek Gods plain, full-fat Greek yogurt, skim milk, and a bit of cream (to turn the skim into whole). Is there something about these ingredients that wold cause it not to work? Should I set it outside where it's warmer? Just add lemon juice and call it a day? I still have over four hours until dinner.