No Material but SAP says RELEASE

Dear gurus,

My customer is encountered a problem of Availability Check. Some of the material they need to release the Prod order are not in stock (we have 0 in actual stock - MD04) but SAP says that "All materials are available". I checked the master data for this items and there is no difference with the other good ones. they all have the same availability check configuration. I noticed however that they posted inventory differences (mvt 711) with WM each month. Maybe the availability check of SAP is based on the inventory levels posted in the WMS.

Please advice

Regards