One of the biggest issues when fishing a nymph, is getting the fly deep
enough when trout are feeding close to the bottom, at the same time,
letting the fly dead drift without showing any pull from the line or
leader. Current is always faster near the surface than near the bottom. So,
when line and leader land on the water, they immediately exert pull on the
fly, unless you remedy the situation with your presentation.
As a result, even though we think we're fishing a fly close to the bottom
with a dead drift, it's not often the case. In most aquatic insects and
crustaceans, when they drift, don't swim or at best feeble swimmers, so
trot off and shy away from a fly that's dragging. But, it's hard to see
drag when your fly is under water.