Opencart affiliate module mystery

For some mysterious reason, the Open Cart affiliate module works in weird manner.
I am using OC 1.5.3
I have tested this aspect in 1.5.1 also.
When I create tracking code and anybody else uses that tracking url, only the first order made has an entry of “affiliate_id” and “commission” columns of order_product table.
Even this entry does not cause to show an entry in my affiliate panel to show the commission.
However, I expected that even in above case there would be some entry in affiliate_transaction table also.
But I am able to make an entry in affiliate_transaction only when entering affiliate transaction from admin panel.
Without that, even if a customer makes purchase on my tracking code, I am not able to see the commission in my affiliate account panel.
Is there anybody who can throw some light how affiliate module is supposed to work?

Thanks