I was born and raised in the Midwestern United States, and always thought that I spoke very clear, easy-to-understand English. But when I go to England, I get the same reaction as you - very often, I am asked to repeat what I've said. I think perhaps the British ear is trained to hear a certain accent, and they have to stop and think twice when presented with a different way of speaking. The same goes in the U.S.; certain British accents (the BBC-style ones) are easy to understand, but a speaker from Liverpool or Edinburgh or Birmingham would probably be asked to repeat himself.
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