If your student tells you they are not receiving the homework you are assigning, it could be because:
they haven't joined your group yet. You can check if they have from the "Groups" tab, by clicking on the name of the group to which they should belong, then on "Edit" and checking if they appear under "Members" on the right side of the screen.
if you see that your student does belong to your group, it may be that they hadn't joined the group yet by the time you created the assignment. To check this, go to the "Assignments" tab of the group where you created the assignment, open the assignment and click on "Edit".
Then, click on the pencil icon next to "students selected", on the right side of the screen.
A window will open with the list of members of the group. Those students who have received the assignment will have a blue tick.
Select the missing student and click on "Done" to include them in the assignment. Finally, click on "Save" on the assignment window to update the changes.