Batch Assignment required

Dear All,

My senerio is this : -

(1) I am creating delivery with batch split ...

(2) Sometimes inspection lot created with batch assignment & I am able to record the result & UD but some times system made inspection lot with full delivery qty and when I am tried to record the result system given an error for "batch assignment required" . I am not able to understand why some system do other procedure & some time do other...

Can anyone help me out...