Flank steak came out chewy this time. What's to blame? I have stir fried flank steak about 5 times now, but this time it came out really chewy.  Is this because I may have cooked it longer than usually?  Or is it the meat or some other factor?

Here is the procedure I followed:


Cut it across the fibers (perpendicular) into 1/4" wide slices.
Marinated the pieces for about 10 minutes in a little soy, wine, and aromatics.
Cooked on really high heat for 1 minutes, stir fried for another 2 minutes. 
Took it out. Put it back in and stir fried for another 1-2 minutes.
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.