When you're on the road, you might be using a pre-ground bag or a friend's grinder set for French press. That coarse grind is fine for a 4-minute hot brew, but for cold brew it needs more surface area to extract properly. If your grind looks like breadcrumbs, the water can't pull enough flavor in 12–16 hours, leaving you with a thin, sour cup.

Instead, aim for something between table salt and raw sugar. If you're buying beans before you travel, ask for a cold brew grind or a medium-fine setting. If you're grinding at a cafe, say you need it for cold brew—they'll know.