Flank steak came out chewy this time. What's to blame?
Perhaps it's because you used grass-fed instead of corn-fed beef. Grass-fed beef tends to be more chewy and flank steaks aren't exactly the most tender of cuts when stir-fried. 

It also seems that the slices may have been in the pan/wok a tad too long, especially when they aren't particularly thick. 

You could try using a fruit-based marinade that includes pineapples or papayas. They're known to have enzymes which break down meat fibres exceptionally well. I've found that they're far more effective than purely acidic marinades when applied to meats.