Why are most of my Timefields blank when editing a node?
Some of the starting pointers I can think of.


Ensure there is no custom code altering the form field. Search for hook_form_alter/hook_form_FORM_ID_alter in your custom modules. 
Check your drupal error logs, if there is any error log related to timefield.
Check your browser console, if there is a JS error.
Check the timefields module issue queue if there is already such error reported, that would be the best place to start from.
Try installing timefields module on vanilla drupal and check if the same issue persist, in that case add this in the issue queue of module.
Make sure all timefields get data when saved for first time, debug using devel module,go to 'devel/php' page and run dpm(node_load('your_node_id'));, see inside node object if those fields have value or not.
"Like s there anyway the possibility to keep the values of the fields somewhere, so I could populate the missing fields via javascript?" Yes, you can only if field values are getting saved to database, in fact simple would be to do at php level something like

function my_module_form_alter($form, &amp;$form_state, $form_id) {
  if($form_id == 'your_form_id') {
    if(isset($form[#node]-&gt;nid)) {
      $form['yourtime_field']['#default_value'] = $form['#node']-&gt;yourtime_field['und'][0]['value'];
    }
  }
}



But this is not the solution, this will be re-attempt of what drupal already trying to do. Best would be to identify why values are not showing by default.