I am adding this because one of the sites Sachin is talking about is my site.
First let me day that I am testing with my personal credit card , so I know for a fact that the credit card is a good account.
I have been testing to find the problem and here is what happens.
If I buy something on one of my websites without updating the plug in the American Express card work perfect every time.
If I buy from the same website after updating the Paypal Plugin the American Express is refused.
I have tried this 3 times now with the same results every time.
So I can no longer accept American Express on any f the sites I upgrade.