It's a 20% chance to learn the schematic each time you RE the item. That means you have an 80% chance to fail each time you RE the item.

Each RE is an independent event, so each time you RE is a new 80% chance of failing to learn a schematic. RNG will make you unhappy sometimes, and other times you'll get that shiny Dread Guard schematic on the first try. Yay random outcomes!