Problem with custom field

I have added a custom field to VBAK table using Append Structure. Then I'm trying to update a value to that field using one of the user exit but it is not updating. If I try to update some other custom field of VBAK which is already there through the same function module, it is updating properly.

It looks like I have to do something else after adding a custom field in order to use it. Append Structure, data element and the VBAK table are all activated properly after adding the field. Please help.

Thanks,

Rakesh.