美国采购需求：圣诞灯饰、圣诞小径灯、照明

I've run into an issue with a string of lights we purchased recently. Out of the 10 bulbs, the last two aren't lighting up at all. I've tried a few basic troubleshooting steps, like checking the connections and ensuring everything is plugged in correctly, but unfortunately, nothing seems to work. Could you please advise on how to resolve this problem? Is there a specific technique or a common fault that could cause just the last two bulbs to fail? Any guidance on how to fix the situation or prevent it from happening again with future purchases would be much appreciated. Thanks!