Fields are not added to view

I can't explain why, but I noticed by debug-outputs that these fields are not added to the view after processing the following lines:
#377, project_forecast.module

/**
 * Implementation of hook_project_forecast_tasks_view_alter():
 * Add our own required fields to the tasks view -
 * assigned user, time need, and project.
 */
function project_forecast_project_forecast_tasks_view_alter(&$view, $display_id, $constraints) {
  $timeneed_provider_name = variable_get('project_forecast_timeneed_provider', '');
  $assigned_uid_provider_name = variable_get('project_forecast_assigned_uid_provider', '');
  $project_nid_provider_name = variable_get('project_forecast_project_nid_provider', '');

  $timeneed_provider = value_provider($timeneed_provider_name, 'hours');
  $assigned_uid_provider = value_provider($assigned_uid_provider_name, 'uid');
  $project_nid_provider = value_provider($project_nid_provider_name, 'nid');

  if (!$timeneed_provider || !$assigned_uid_provider || !$project_nid_provider) {
    return FALSE;
  }
  
  value_provider_add_fields($view, $display_id, $timeneed_provider);
  value_provider_add_fields($view, $display_id, $assigned_uid_provider);
  value_provider_add_fields($view, $display_id, $project_nid_provider);

  //debug output here :)
  dsm($view);
...

Regrettably I still don't know how to fix, but I though it's worth to create an issue for that,
greetings,
sevi