Why card reader doesn't start reading card after 3 or 4 cards process?

I'm working for an intelligent card solution company which is providing intelligent ID cards for different companies. i have developed the application which processes the card data ( MOC, Activating, changing PINs ).

The problem is :

After changing PINs and delivering card to customers by doing MOC, card readers doesn't show any reaction or interrupt for the next card after each 4 or 5 cards (or sometime simply for each next card) . we generally use Dermalog ZF1 combo (reader,scaer) and futronic combo devices.

What i did:

I tried to do warm reset and cold reset after each card and it didn't work. Changing windows and computer main board had no effect... Changing driver didn't worked... there were no change by ruing in 32bit and 64bit windows...

Clues :

When we put next card immediately after previous one and there is no delay processing new card it works good... When i disable and enable card reader from device manager it starts working again.

My solution:

I know it's somehow fuy but it is good work around solution and works well. I added some codes to my application which makes device disable and enable programmatically and after that card reader starts working again.

My Question:

does anybody knows what is the problem? is that a CCID bug or hardware problem? or is there any problem in USB CCID driver which is causing this problem? does USB hubs idle state have effect on functionality of card reader? I have tried lots of things which I have mentioned in "What i did" part and nothing changed... any kind of help would be greatly appreciated.

thanks in advance...