Lm_paypal_subscription problem: nothing happen to role assingment when paypal return back

AoA to every one.
i am new to drupal and i have a problem to paypal subscription. i want to create paid membership, for this i create all steps successfully but lm_paypal cannot assing role when user subscribe successfully. i check thal all variables available in post. so what i can do to add role. can any body help me. thanks in advance.