Cat eye gel lines usually disappear because the shimmer was not locked in quickly enough. The magnet shapes the particles, but curing fixes the result.
Common causes
The most common issues are a thick gel layer, a weak magnet position, moving the hand too soon, or waiting too long before curing.
Simple fix
Use a thinner coat, bring the magnet closer, hold it steady for a few seconds, and cure as soon as the line is crisp.