getCollection works but getMany doesn't

<?php
	$parent = $modx->resource->get('id');
	$count = 0;
	$criteria = $modx->newQuery('modResource', array(
		'published' => '1'
		,'deleted' => '0'
		,'context_key' => 'web'
		,'parent' => $parent
		,'class_key' => 'modStaticResource'
	));
	
	$criteria->sortby('pagetitle','ASC');
	$criteria->limit(16,0);
	$staticResources = $modx->getCollection('modResource', $criteria);  

	foreach ($staticResources as $staticResource) { 
		//$accessGroups = staticResource->getMany('modResourceGroupResource'); //can't understand why this line won't work but the line below will
		$accessGroups = $modx->getCollection('modResourceGroupResource', array('document' => $staticResource->get('id')));
		foreach ($accessGroups as $ag) 
			{
				echo $ag->get('document_group');	
			}
		$outputArray['videoRelId'] = 'overlay_' . $staticResource->get('id');
		$videoPath = ($staticResource->get('content') != '') ? 'assets/media/' . $staticResource->get('content') : 'default.flv';
		$outputArray['videoPath'] = file_exists($videoPath) ? $videoPath : 'assets/media/video/default.flv';
		echo $modx->parseChunk('listSelectedVideosOverlay', $outputArray);
		$count++;
	} 
?>

$accessGroups = $staticResource->getMany('ResourceGroupResources');