Most students (even the youngest ones) have time constraints after school, especially when it comes to getting homework done on time. After a while, you'll get an idea of who always does their homework and who rarely does it. Have a meeting with those students who have problem getting the homework done to see if there are problems at home such as: too noisy, uncomfortable place to do homework, etc. If this seems to be the case, you may want to make an exception and have them do the homework during class time when they first enter in the morning, or after they have completed other work. Although this is not ideal, they will have the opportunity to independently get their work done. This will also give you a chance to see if the work is not getting done because of home conditions or because the work is too difficult or not understood.