Moving average price calculation in MBBS transaction

Hi Experts,

I have a weird situation, when i scrapped a material with project stock (551 Q), the system is not generating the accounting document for the corresponding material document. I found that MAP (moving average price) for that material/WBS element is zero in MBBS transaction. can anybody explain how the MAP is calculated for project stock and what could be the reason for not generating accounting document for 551 Q movement type.

Thanks in advance.

Regards