Why there is an occasion merchant backend did not receive transaction notification?

The program on notification page file (parameter notify_url corresponding page file) did not complete execution due to program throws an exception, not able to visit page file and other reasons.
How to check:
Check server asynchronous notification page (parameter notify_url corresponding page file)
Step 1: Check notify_url page file
A. Merchant and Private Key are filled in correctly, not space is required
B. notify_url path is accessible on internet, ensure the page is not blank or no error message prompt by the software itself
C. Request parameter show_url, return_url, notify_url should not consist of self-defined parameter, such as:
http://www.xxx.com/return_url.asp?id=123?id=123,the parameter follow after “?” is self-defined parameter.
D. Check the coding format
Coding format error. May use log function (log_result) to record data in Chinese to check whether there is any messy code. If there is messy code, it is due to different coding format setting. Check the coding format parameter input-charset, webpage attribute code, overall code for web application, etc. ensure no messy code when transmitting data.
E. In notify_url page, only enter SUCCESS and ensure the page is entirely blank without any HTML label, space, crlf and other characters.
SUCCESS code for different programming language:
ASP:response.write "SUCCESS"
F. No seesion or cookies should include in notify_url page, do not auto redirect page
Step 2: Check server configuration (Below is the guide for checking parameter)
A. Server write permission is granted or not
B. DNS resolution, whether enough to resolute Dinpay IP
C. Is there any firewall
D. Allowing ports 443 or 80 or not, and it’s not in stealth mode or not being occupied
E. Any changes on server IP address, if there is change, need to rename notify_url page file, and update the parameter nofity_url value.
For the case where merchant website has process multiple (such as pay once but repeat multiple top up) process in one transaction, after fixing merchant website’s program or server according to the solution, must ensure page redirect to synchronous notification page file (return_url) and in server asynchronous notification page file, before executing programming code for merchant’s business logic, must determine whether the process has go through similar process (which the transaction payment is made, the order on merchant website has been updated or not), such as: whether the member has top up once or not. Following execution can only be executed when no similar process is done