Store Credit Not Being Deducted on Some Orders

One of our clients gives a $5 store credit to newly registered users. Unfortunately this $5 credit isn't being deducted for some customers despite applying it towards purchases. Allowing them to re-use the credit.

In the customer_transactions table I can see some orders being processed correctly (deducting store credit after purchase), but not others. I've seen similar issues like this posted a while back with no solution.

http://forum.opencart.com/viewtopic.php?f=115&t=52022
http://forum.opencart.com/viewtopic.php?f=113&t=92518

I haven't been able to find a pattern why some orders work and others don't.

Any help would be appreciated. The version of OpenCart is 1.5.5.1

The store credit mod simply credits their account with 5.00
The actual checkout system that deducts this credit is a core feature and I've never seen it fail. But I think you have to have the order hit the same order status as the "Completed Status" in the system settings for it to deduct correctly

So credit deductions take place when order status changes?

Currently the client only changes order statuses to as far as "Shipped", doesn't seem they ever marks them "Complete". Which I would say is the issue - except like I said above some orders are working and others are not.

Thanks for your help. For now the client has remedied the situation by manual managing store credit via OC's core functionality, but I am hoping to find the real issue. I'll post here if I find it.