It sounds like you database may be corrupted. This could happen by not having your fields numbered consecutively in your .cfg file.
It can also cause problems if your html_record_form doesn't have each and every field listed, either as a visible or hidden field. If you make changes, you have to also change the database file.
If you can't find the problem after checking your .cfg file, you will want to save a copy of both your .cfg and html.pl file as .txt files and provide a link to both .txt files so they can be viewed online.
Unoffical DBMan FAQ
http://webmagic.hypermart.net/dbman/