Form correctly submitted and saved but edit fields not populated

I'm developing a contract management module for Drupal which needed separate database tables, given that those present in the default aren't enough for defining contracts and contract lifecycles. Anyway, I've defined a new table and a new module. The new module is working as I want it to ... the preview is ok and the form_validate is ok. However, when I want to edit the form with http://path/to/drupal/?q=node/X/edit, I see the correct form (the same form as when I add a contract) but the fields are not prepopulated. If I fill information in and submit, form_upload correctly performs the update on the dbase tables. This isn't much good, however, if the user cannot see the pre-existing values in the edit modus.

What hook is responsible for displaying the pre-populated form and does anyone have an idea what I could be doing wrong here?