combo box in access

How do you display a list of projects that are assign to one person. My SQL is like this:
SELECT tblProjects.ProjID, tblProjects.ProjName
FROM tblProjects INNER JOIN tblAssignments ON tblProjects.ProjID = tblAssignments.ProjID
WHERE (((tblAssignments.PersonID)=[cobPersonID]));
The problem is I cannot add any data for other people except for the first person that automatically appear when you open the form. It's because that the project list won't change its list when you select another person's name