Change Rules for Line item

Hi,

I have customized the change rules for Line item.

Still the system not allowing to make changes to that particular field? Can any one tell me, whats the reason for the same.

Following Field is made as changeable for the line item.

BSEG-VBUND Trading partner.

The above field is available in More data Tab or additional data tab while making entry or display of document.

Thanks,

Lalatendu