Well, I am also in similar situation. I was in Uruguay (South America) for about 18 months and i have tried different ways to get PCC from Uruguay but have no luck.
But I agree with one of the comment which says we need to submit PCC from all countries where in our stay is more than 6 months since 18 years of age.
I am sure, many would be in same situation and i think some of them would have managed to get PCC from USA.