Robert_Charlton - 5:20 am on Apr 30, 2012 (gmt 0)
US Google serps are generally more competitive than foreign serps.
Inbound link location can also affect ranking in different geo-areas. Check to see whether your inbound links are coming from the US. If too many are from out of country, that could adversely affect US rankings.
If your site is hosted outside of the US, that might also send signals to Google that could hurt US rankings.