Flank steak came out chewy this time. What's to blame?

I would suggest from the information you've provided so far, that the most likely cause is that you've overcooked it. There are a couple reasons that might have happened.

Pan

• What pan you choose matters here. If you don't have a powerful stove (ideally gas for stir frying; or induction with the right sort of wok adapter), then a flat stainless skillet is your best bet. If you have the right stove, then a thinner wok is ideal as it'll recover heat more quickly.

Heat

• How high is "really high"? Do you have gas? induction? crappy old electric? Not all stoves are made equal. If you're stir frying something (especially something like flank steak). You want to cook it very very, hot.
• You also want to cook it quickly. Don't crowd the pan.

Assuming your average electric home setup:

1. I'd use a flat bottomed stainless steel skillet. Let it heat up dry until it's super hot.
2. Drop a droplet of water onto it, if it beads up and dances around you're good.
3. Add some oil.
4. Add a single layer of meat until it's cooked. If you're stove isn't hot enough, you're not going to recover heat as quickly as a pro range. Let it cook a bit before flipping it over.

If you happen to have a powerful enough stove, then a wok is your best bet. Same deal heating it up and adding oil. This time, you can keep it moving till it's browned. I'd take it out as soon as I didn't see any red.

With both cooking methods, I wouldn't add it back to the heat except to mix it with everything else. (Cook meat and remove. Cook vegetables and remove. Stir fry noodles if desired. Throw everything in, add sauce and combine and then remove).

Cut

• You've said you cut it across the grain, so I'll assume this isn't your problem. It's worth reiterating for anyone else's benefit, as this is the most common problem.
• Depending on how sharp your knife is, you could try going even thinner than 1/4". If there's any chance your 1/4" is more than a 1/4" that would be a negative, as it's going to toughen up as it'll take too long to cook. If you have trouble cutting it thin enough, you could freeze it for about 30 min to an hour before cutting. It's a lot easier to get thin cuts that way. If it came frozen, just cut before fully defrosted.

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.

You bought a bad cut of flank steak. They used to be tender and flavorful and my experience is in the past year or so the toughness and lack of flavor in flank steaks has gotten to the point where I won't even buy it anymore. I have switched to flat iron steaks for what you describe. Much more tender and flavorful.

And people, you really can't overcook a flank steak to where it becomes tough and chewy. It has to start out that way...