There is no requirement that every document contain values for every field. Doc A could have fields z, y, x, and Doc B could have fields x, w, v. So, when you say "some of the values are being returned as null", do you mean that you *never* get any values for some field or you get values for a field for some, but not all documents?
You might try using Luke to look at the specific document that's giving you problems and see if it has values for all fields. You can enter the document ID (i.e. Lucene's internal doc.id()) in the search field and examine the actual doc (sorry if you know that already <G>). I think it's the second tab, titled "document..." If you never get values for some field that Luke shows is in the index, the first place I'd look is whether have the field name correctly entere (including case). Hope this helps Erick