Thanks to the payment log, you will be able to read several data about your payment plugin process and some error create by the payment process.

It's a file where are write some important data while processing with your payment plugin, so you will see what happen in backwards, and let's hope so find trouble roots!

For find the payment log, go to components -> Hikashop -> Configuration.

 

 

And in configuration, in the Main Tab, click on Files

 

 

Now, you can see the "Payment log file, and this path, to read this log click on "See the report"

 

 

Hope this will be usefull for find what's wrong with your payment plugin!