Apex test class - trigger not firing

I wrote an apex trigger to update a field (status_update_date) when an account's status is changed.

In the sandbox, if I change an account's status, the status_update_date successfully updates.

When I run my test class, the trigger does not get called and I get a null value for the status_update_date. Any ideas why the trigger is not called? Thanks!

Code below with trigger commented out at bottom of code:

    @isTest
    private class UpdateStatusTest{
public static Account A;

public static String initStatus = '';
public static String finalStatus = 'Fully Active';

static testMethod void testWithAccount() {

    A = new Account(
        Status__c = initStatus,
        Name = 'TestName'
        );

    insert A;     

    test.StartTest();
    A.Status__c = finalStatus;

    update A;
    test.StopTest();
   // System.assertEquals(finalStatus, A.Status__c);
    System.assertEquals(system.today(),A.Status_Update_Date__c);

}
}



   //  trigger UpdateStatus on Account (before update) {
   //for(Account a: trigger.new){
        //If the status has changed, update the date
   //     if (trigger.oldMap.get(a.Id).Status__c != trigger.newMap.get(a.Id).Status__c) {
    //    a.Status_Update_Date__c = system.today();
    //  }
  //}
//}