Need Help on User exit

Hi,

this is my requirement: When create a shipment (VT01N ) transaction code , i need to do some calucation and finally update into field (VTTK-EXTI1). but it is not updating: iam using FM exits :EXIT_SAPLV56U_004 and EXIT_SAPLV56U_005.

data is coming into internal table , but not upating into field.

please help me.