As school went on, there was an opportunity to get your homework done in class if you had completed the seat work already. That is a great incentive.
I have seen a teacher not grade homework, but give out classroom infractions for not turning it in. So that way is kindof punative.
Another class I have seen the teacher expect homework to be turned in, and then assigning an actual grade to one random homework assignment per week.
Especially in middle school, each teacher has a different style. And then there are district wide policies that may come into play as well.
The work also is more interesting in middle school, so that should help some, too.
