Goods issue quantity is not/only partially confirmed

Hi All,

I am unable to do goods movement for Parts in a Sale order though stock exists under Unrestricted use and Available quantity due to error 'Goods issue quantity is not/only partially confirmed'.  When I go to Parts tab and carry out availability check,then I am able to do Goods movement without any error.

This is happening very rarely. Can anyone explain what can be the reason for this?  

Regards,

Manasa