It may fail to turn the fly over cleanly. A tippet that is too light is too soft: by the time the energy reaches the fly there is not enough left to straighten the leader, and the air resistance of a large fly pushes back against the line. The leader lands in a coil, and the fly may spin and twist the tippet instead of unrolling straight.

The fault is in the pairing, not in the fly or the fish. Tippet choice starts with the fly, not with the expected weight of the fish: a large, air-resistant pattern needs a heavier, stiffer tippet to support it. If the tippet is fine enough compared with the wire of the hook eye, the knot can also slip instead of holding.

So when the same hook size casts cleanly with one fly and lands in a heap with another, look at the tippet before you doubt your stroke. Go one step heavier and let the fly turn over. Landing softly is the next problem to solve, not the first.

Source

From the Orvis tippet chart, on diameter, stiffness and air-resistant flies; see also leader and tippet; terms follow the FFI Fly Casting Definitions.