I have seen this question rasied before. 1st, what build are you using for ORM, code generator? If you haven't updated your libraries in a couple months there have been many revs.
2nd, Add a second "dummy" field to the table, re-gen the code and see if the constructor is formed. I think there was a issue with single column tables in previous versions.
3rd, search the forums for "constructor" and see if any of the results match your issue.