Accounting commitment document missing

Hi,

I created purchase order from purchase requisition with Account assignment category A - Asset and I found ambigous behavior of AC commitment document.

When I created purchase requistion (for Asset) than no AC commitment document was created, subsequently I create purchase order and AC commitment document was sometimes created, sometimes not....

When I created directly purchase order (without purchase requisition) than AC commitment document is always created.

For Account assigment category e.g. - K -cost center has AC commitment document already created after purchase requisition creating.

Any helps.

thanks

Petr