Availability check - how it works - sales order

Hi,

We have a situation, there is enough stock for a material and the requested quantity is way less. The schedule line delivery date is 11/18/08, but the confirmed date shows only 11/21/08. The scope of check is, inlcude deliveries and check without RLT is checked and all others are not checked. If I try to change the Schedule line date for availability check to say 11/20/08, SAP comes back and says on 11/20/08 only 0 quantity is available. But as per MMBE, there are more than 100k is available. Can anyone explain, what is going on or what else is taken into account during availablity check? Thanks,

Regards,

Sundar.