DataTable sortable not woking in some columns;

ananis

I have a table about dataTable ,there has one column I set the data 'null',then render it with the full data's compute result ;And I set this column "sortable:true',however,unfortunately,it don't working;some time,it working ,but,even if working ,the order is disorderly,It is no positive sequence or reverse order ;who can tell me what's reason ,thanks;