cant get field STCD3 from lfa1

Hi im using the logical tabla kdf i need the field STCD3, i try to get it from the

get LFA1.

MOVE LFA1-STCD3 to i_lfa1-stcd3.

But the value is ##################.

in the table lfa1 with the same lifnr it does have the correct value,

i have already solved like this,

get LFA1.

select stcd3

from lfa1

into lfa1-stcd3

where lifnr eq lfa1-lifnr.

endselect.

MOVE LFA1-STCD3 to i_lfa1-stcd3.

Still i want to know why some fields have ##### instead of the value.

Thanks

Pablo Santos