A sharper cat eye nail line usually comes from three basics: a thin magnetic gel layer, a steady magnet distance, and curing as soon as the shimmer is where you want it.
Work one nail at a time
Magnetic particles move best before the gel starts to settle. Apply gel to one nail, hold the magnet close without touching, then cure when the line looks clean.
Keep the distance repeatable
If the magnet is too far away, the line will look soft. If it touches the gel, the surface can mark. Practice on tips until your hand position feels natural.