Custom Subscreen in Service Master AC01 not invoked

Defined custom subscreen(SAPLXBAS 8100) for service master AC01. User Exit is activated , however the line of code inthe main screen which calls the subscreen is not being called at runtime. Details are as follows

ECC6.0

UserExit : SRVASMD

Calling screen No. Area Called screen No.

SAPLBAS0 0300 ZUSATZ SAPLXBAS 8100

this is the line which is being ignored while executing the transaction ac01

CALL CUSTOMER-SUBSCREEN ZUSATZ INCLUDING 'SAPLXBAS' '8100'.

Any inputs appreciated and points would be awarded