Substitution for Business Place at item level not working

Hi Experts

I have written substitution in OBBH for BUPLA Business place but it is not working. My pre-requisite is If company code = X & With holding tax GL -> BUPLA =1140.

Requirement is : Whatever may be Business place in FB60 at header level, client wants With holding tax / TDS GL to have BUPLA=1140 only. So I am writing substitution for this GL account.

But system is copying BUPLA entered at header level, to BUPLA at item level. Please advise on how I can achieve this.

Regards

Kapil