How to get third graders to do work???? HELP!

I am at a loss! I have tried different things different things to get my kids to turn in their work. I seem to have the same few kids who don't turn in work and then the work that I do get from my kids is either all wrong or poorly done. I think that I am going to ask other teachers for examples of high quality and non high quality work and we are going to do a lesson on that in my class. I don't feel like the work that I give is hard. I feel like they just don't care and I don't know what to do. I send home missing assignment alerts for those who just don't turn in the work. I let them redo work that is less than a 70 for a grade of up to a 70. I feel like I am more than fair but am I being too nice? What do the 4th grade teachers do? Am I thinking that they should have more responsibility than what they should? I don't know what to do. What do you do in your third grade classroom or what advice can you give for this issue?  I need help! Let me know what you think!