"The edit function also does the same." I'm not sure I understand this sentence. The edit button does not appear?
Which version of Moodle are you using? And do you know which theme?
Meanwhile, do you have the same role in all three courses? Non-editing teachers are, obviously, not allowed to edit.
Are you using groups? In the 3rd course, you might not be in a group, or you might be the only person in a group.
It's also possible that you do not have editing permissions even though your role is teacher. to check that in Moodle2+, you would go Administration > Course Administration > users > Permissions > Check permissions and use the Advanced role override dropdown to select Teacher. Note that if a permission has been changed, it will look like "Teacher(1)." If nothing has been changed, it will look like "Teacher(0)."