A dead drift is fly fishing's central skill. The fly should look like debris — a leaf, a dislodged nymph, a spent insect — moving at the water's speed, not yours.
If your line forms a downstream belly that tugs the fly across the current, you're dragging. Trout refuse drag almost every time. The fix: mend the line as soon as it lands, and reposition until the fly floats at exactly the speed of the seam it's drifting through.