When using the destiny knot, the remaining stat is random.
You're doing it right, but the odds are against you, and you're just having bad luck. Keep using the destiny knot and the lapras with multiple IVs. Eventually the Def will pass down. Even if not all of your stats pass down, if you get Def and even one of the other stats you're looking for, switch that one with the Def parent, if they're the same gender. Two parents with a stat are more likely to produce an offspring with that stat. Then just keep trying and switching for better parents. Good luck!
X FC, Xantara: 2449-5027-2210 (FS: Abra, Espurr, Gothorita)
White FC, Onyx: 5286-8494-9103
Complete Shiny Pokedexes:
Progress on complete shiny Kanto pokedex: working on the Magnemite line