What does an employee record expired on Assignment Rule Employee mean?

We have some Order records which fail to get automated employee assignment. We have rule 1 and rule 2. Rule 1 has a sequence 1 and Rule 2 has a sequence 2.  Rule 2 is the overflow assignment rule for Rule 1.   The order is supposed to get the employee assignment from Rule 2, however, order did not get any employee assignment.  Here is my observation:

Order met the all criteria in Rule 1, but the only employee record is expired in 'Assignment Rule Employee' applet under Assignment Rule - Rule 1.

I wonder is it because order met the rules in Rule 1 , but employee is expired, so there's no employee assignment nor overflow to use Rule 2? Please advise?