Unable to create order line

Hi,

We have some orders which contain service items. These items were define wrong and defined as inventory, shippable items.

Orders were created.

Later on lines containing these items cancelled.

Items modified as non-inventory and non-shippable.

Now when we are trying to create new order line sysetm is giving following message;

" You are not allowed to create order line becuase Header level invoicing has completed, cannot insert new line."

These orders are very important and we cannot create new orders and we have to add lines to these orders.

Any quick help is highly appreciated.

Regards,

Syed Iftikhar Ahmed