Calling BAPI_APOATP_CHECK for custom field

Hi,

We have an enhancement where we have added a custom field in table VBAP.

During sales order change which is being done via a custom transaction, we want to call BAPI BAPI_SALES-ORDER_CHANGE and try to pass the custom field and in tern we trying to call BAPI_APOATP_CHECK.But we see system doesn't call BAPI_APOATP_CHECK if we change the custom field.

But if we change standard values like plant or storage location via this custom transaction, system calls BAP_APOATP_CHECK.

So not sure why system doesn't call BAPI_APOATP_CHECK if custom field is changed.

Any clue?

Regards,

Dipak