Issue Binding

If any production order has been created on basis of a Sales Order using Order-> Planning. In any case system will reserve the output and keep order tracking is none, for this Sales Order even if reserve is selected as never or order tracking is none. 

Reason : This is called Binding in NAV, if u click on order reservation, then you can notice that Binding updated by system will be Order-By-Order. In context to this order system will not allow you to do forward scheduling as well.

So if u get any error Item is not in Inventory though still it is visible one of things you can check (in Manufacturing environment) through order that it is not Binded.