Left and right misses are tracking problems, not aiming problems. Tracking is the lateral path of the rod tip during the casting stroke; when it curves sideways the fly leg curves with it and the fly lands off line.

• Find the body motion. Poor tracking is often caused by twisting the wrist or rotating the shoulder or body during the casting stroke; a fly leg curved right or left mirrors that curved tip path.
• Aim the reel. Keep the reel in one plane and pointed at the target throughout the casting cycle; watching reel orientation catches a sideways tip path.
• Check your alignment too. The 180 Degree Principle asks for the fly line and rod tip path to be aligned with the target at the start of each stroke, both laterally and vertically; line swung out to one side gives a curved fly leg.
• Read the result, then drill it. Targets tell you whether the fly landed too far left or right, and tracking errors are fatal at the 30 and 45 ft targets. Cast at 45° above a tape measure: a straight tip path lands the line alongside the tape, not feet away.

Source

From the tracking faults and target work in the CI Study Guide (2025) (PDF) and the Horizontal Casting Drill in Appendix A; terms from the FFI Fly Casting Definitions (Tracking, 180 Degree Principle). See also Task 12, accuracy on-side and Task 13, accuracy off-side.