Missing column values in export dataport

Hi, I have a strange problem. I have built a very simple export dataport to extract certain columns from a table into an external textfile. The odd thing is that one of the columns always comes back containing 0 (even though when i look at that value held in the database it has a non-zero value). The source table has 75 columns in it, do dataports have a problem when they are working with tables that have this many columns?

Any ideas?