I made a gravy with RICH homemade turkey stock and a DARK roux of cake flour and vegetable oil. It was a lightly thick gravy still on the stove's low burner. I wanted to thicken it a bit more but had no more roux left. So, I mixed a tablespoon of gravy and cornstarch in a cup on the side, and then whisked it all in. The gravy then took on a perfect thickness. Happily, I walked away for 20min, leaving the gravy at a VERY low simmer. When I returned I found it had turned as thin as water! Any idea what happened? Do flour and corn starch not play well together very long?