[eluser]SimonH[/eluser]
Hi,
the reason you are having this issue is that you are using the set_value function provided by the form_validation library to generate the values to be inserted into the DB. You will only be able to grab input for fields that have validation rules because set_value is originally meant to be used to repopulate fields with their original values to save the user from having to retype their information. In this context may I suggest using the input library to grab the information you need such that a textbox with the name "username" will be available through the following method call $this->input->post('username'). Don't worry the value returned through this will still have your validation rules applied to it.
Hope this clears up the issue.