Purchase orders item texts incorrect display

Hi everyone,

i am having a bit of a hard time solving an incorrect display of purchase orders item texts.

For an unknown reason, i can't manage to modify the item's text of a purchase order, it's as it has been set on "display".

I checked AKTH, ME21,ME21N and PT0F( the item i use) on spro :

Materials Management=>Purchasing =>Purchase Order= > Define Screen Layout at Document Level => Reference data item for the texts and it's always set on Optionnal Entry.

I even tried to set them on Requested Entry but i didn't manage to get the text-item field modified.

Any kind of help would be appreciated.

Thanks.

Siidou