So basically you have to figure it out what error is happening
Error Can be following
1. PHP ERROR
2. Apache/Nginx Error.
3. Magento to Payment Gateway communication issue.
4. Error in the payment gateway side.
So you can do following
1. Check Magento log file(Location: var/log/)
2. Check any error is reported in payment getaway side.
3. Check Apache/Nginx log files for further details.
4. Check your request is send successfully to Payment gateway.
5. Check the request parameter send to the payment gateway.
Suman Kar(suman.jis@gmail.com) Magento Certified Developer Plus Skype: sumanphptech Problem solved? Please give 'Kudos' and accept 'Answer as Solution'.