hi ankita..
i agree with aasif sir that it is because of the bidirectional transmission.For simulation purposes you have to introduce delay for the working of bidirectional fibers on optiwave by using delay element and sweep iteration and index value.
you should refer lesson 5 as the bidirectional topic is discussed in it thoroughly. i hope you will clear your doubts from his lesson.
Also attaching a link for bidirectional-optical-fiber in whcih delay aspect is discussed…..
with regards