Validations using OB28 is not working for GL posting using FB01

Hi Experts,

Validations using OB28 for posting using FB01 is not working.

My requirement is after entering a line items and click post validations should be triggerd.

i want to block a posting based on document type.

i have maintained as below..

Prerequisite.

( BKPF-TCODE = 'FB50' OR BKPF-TCODE = 'FB01' ) AND (

BKPF-BUKRS = '1000' OR BKPF-BUKRS = '0001' OR

BKPF-BUKRS = '0002' ) AND BKPF-BLART = 'XX'.

check.

Used userexit to check sy-ucomm = 'BU' (for posting).

entries are maintained in OB28 for relevant company codes.

it is working for FB50 but not FB01.

same thing i tried to maintain at header level as well as item level. but it is working only for FB50 but for FB01.

Please help me.

Regards.

Venu.