Hi,
For the Proforma Invoice type check what is the TEXT PROCEDURE assigned at both Header and Item
and in VOTXN, check whether the TEXT ID for Material sales text is available or not
and check what is the access sequence assigned to that TEXT ID
and for the Access sequence, whether VBBP TEXT OBJECTS added or not
If everything seems fine, then from the Billing document
from Item text, click on DISPLAY LOG
which will take you to the analysis of TEXT DETERMINATION
in that screen you can find the error if any
regards,
santosh