You should check if you are ranked within the top 1000 pages on google.com (add &num=100 to the URL to see 100 SERP results at a time). If you are ranked just not in the top spots, as Irving said you are merely not geotrageted for the US in the eyes of Google.
However, if your site is not in the top 1000 spots it is likely because of a specific google penalty.