I don't think it's that Americans have particular prolems with irony, but my impression is that they need to have it confirmed that it is irony before they read it that way (eg "smile when you say that"). Back here it tends to be the other way round, remarks are likely to be taken as meant ironically, unless you actually qualify them with "and I mean that" or some equivalent indication.

"That's a nice hat you're wearing." "Why, what's wrong with it?"

"I had a lovely time" "You always complain about everything!"

"Tony Blair is doing a great job" "I suppose you think you could do better..." (And in that example, the reply is of course meant ironically as well.)