Which IVs the Destiny Knot passes is left entirely up to luck, so it might be passing the IVs you don't want constantly. Best thing to do is just keep trying.
There are also the Power items, such as the Power Anklet, Power Bracer, etc. which are useful for breeding. Each one represents a stat, and if you equip the item on a Pokemon, the IVs in that stat will be passed down 100% of the time. So if Swellow or Ditto have IVs in a statthat you're particularly interested in, put the correct item on it and you'll always get the good IV in the stat. Of course, make sure the Destiny Knot is still on one of the Pokemon since it is going to be more useful in general.