Dear Sandile,
What I have understand from your query that; you have paid advance using F-48 and when you execute FBL1n, after giving all the possible parameter, it does not show on the line item even though you have selected the Purchasing Doc from Screen layout.
If this is correct kindly let me know that have you filled the purchasing doc field in F-48 while giving the advance.
I also face this scenario at one client for that I have maintained the Purchase Doc field with the Purchase order number and then in the execution it show up.
Please find the field in attached screen shot this might resolve your query.
Thanks & Regards
Arif Billah