Since I obviously do not know your children, I 'll just toss out a few thoughts that may or may not apply. Different things click with different personalities, and even then, different times....sigh.
A quick look into Proverbs will confirm the fact that a fool cannot learn. There have been times when I have called off a lesson by saying, "You are being foolish and therefore you cannot learn and so I am wasting my time." I then send them off to do some chores and return when they are ready to learn something.
I have also gotten right down in their little faces and gently asked, "Can we agree to seek the mind of Christ on this matter?" Then I am responsible to follow-up with a chance to discuss it with them.
Also, we purpose to give them plenty of opportunities to explore some of their own interests; that can really spark learning. I will allow them to select their own research project, however silly it sounds to me. We set up goals and a timetable for completion to keep it moving along. One current project is the history of the Ford(?) Mustang. My only 'extra' requirement will be to include a timeline that has a bit of world history and church history noted on it.
A talk about just who is the one that is graduating was helpful with our oldest. Once he internalized the fact that I was happy to just keep schooling him and it was up to him to graduate, he seemed to wake up (though it took a number of these 'talks' through the highschool years).
I think a habit we homeschool families can fall into is changing/dropping the assignment when things are not completed. We like to be flexible, but we often are training our children in a lack of initiative and an abundance of procrastination. One thing that truly helped us was to require the child to carry the full weight of whatever day/season it was, while still catching up on assignments. We had a full summer of this one time where the child had to carry his full summer responsibilities, which are numerous on a farm, while using his free time to catch up the school work. He was one tired puppy.
As always, our own attitudes will rule the home. As we look to the Lord for His peace and strength, it will enable us to calmly see the needs in our child's character and apply the proper consequence in a loving fashion.