Paypal IPn not working in website

Question

hello,

in my asp.net application i use paypal for payment its all thing working fine
but IPN not working in website i mean i am not get IPN info from paypal
but if i test IPN.aspx in IPN simulater its working fine coz in IPN.aspx i maintain a log.txt file or database entry its working fine
from IPN simulator but when i pay from website and its all thing workin but i am not get anything in IPN.aspx

so anybody give me some idea or info about IPN
thanks