I agree with Tommie, you probably need to increase your bids significantly because you likely have stiffer competition within the US market. However, I would advise that you increase your daily budget to avoid the throttling of your impressions. If you lower your daily budget you are likely to pay higher per click costs due to the throttling issue. Just monitor your results and adjust the bids until your average daily ad spend aligns with your desired daily budget. A managed campaign typically performs far better than a set-and-forget approach.
Banner blindness may occur over time, I’m not convinced that is your problem. I would definitely run alternate ads on the percentage of your placements. You need to be testing rather than guessing. You’ll never know if that next banner design is the one that doubles conversions unless you are always testing.