Trying dynamic checkboxes population

Hi. I have this code for population a group of checkboxes:

add_filter("gform_pre_render_7", "pre_survey");
function pre_survey($form){
   global $wpdb;

   $strSQL = 'SELECT name FROM wp_pod_tbl_treatment1 ORDER BY name';
   $tmts = $wpdb->get_results($strSQL);
   $i = 0;
   foreach($tmts as $tmt) {
         $i1 = $i+1;
         $id = '10.' . $i1;
         $form['fields'][5]['choices'][$i]['text'] = $tmt->name;
         $form['fields'][5]['choices'][$i]['value'] = $id;
         $form['fields'][5]['choices'][$i]['isSelected'] = ($_POST['input_10_' . $i1]);
         $form['fields'][5]['inputs'][$i]['id'] = $id;
         $form['fields'][5]['inputs'][$i]['label'] = $tmt->name;
         $form['fields'][5]['inputs'][$i]['name'] = $id;
         $i++;
   }
   return $form;
}

The problem is some checkboxes seems not to be stored or even kept when I go to next page and then go back to this page. Am I doing something wrong ?

Also, I would like on next page to show a question for every marked checkbox. Is this the correct hook where I can do this ? Is there any example in how to achieving this ?

Thanks.

Thanks for your answer. Yes, I had already seen this. In fact I think my code is very similar, but I think there is something wrong because some checkboxes are kept and some others doesnt.

Please try this page: http://absoluteme.co.uk/survey/

and then try to mark checkboxes on the right column from "Sculptra" to "Thermage" (you should be registered to make this, but it's free). Then press "Next" and then press "Previous" to return to the same page and see which checkbox values are retained and which aren't retained (since "Slim Lipo" are not retained). Maybe there is a maximum checkboxes limit on a group since then values are not retained ?

Thanks.