instruction after yield return new WaitForSeconds is not working

my program is, when the user click the mouse button, firstpage game will be disappear and secpage game object will appear on the scree, however when i write the below code, all the instruction after yield is not working. can somebody please help me. thank you

IEnumerator OnMouseUp(){

	st1.Stop ();
	FirstPage.SetActive (false);

	yield return new WaitForSeconds (3.5f);

	SecPage.SetActive (true);
	m1.clip = AudioInstr;
	m1.Play ();


}

I think making OnMouseUp() iEunmerator-able is not advised. Have this function call your own function which has the yield in it.