It's not unusual for your magnet to get stuck; sometimes on a heavy object (like a metal pier column) or wedged between some rocks. When this happens, try these solutions;
Try pulling firmly and steadily but not with all your might to assess if you're freeing it or making it worse.
Pull from different angles (for example, the other end of the pier or the opposite side of the boat launch.)
Put a lot of slack in your rope and let the current (if there is one) work for you, moving your line into a new position.
If it’s safe and you can swim, go in after it! Some snorkeling gear will make this a fun and relatively effortless endeavor. You can actually pull yourself down the line directly to the magnet to visually assess the situation and free your magnet or you can stay on the surface and swim your line over and then past the point of hang up and free it from a different vantage point.
If all else fails, you might try a more aggressive approach with a bumper winch or come along. But beware, this can possibly stretch your rope to the breaking point.