I had this problem for many times that sometimes the “find and replace” in Excel is not working.
Actually, this is not Excel’s bug. It is probably due to the encode of the text in Excel cell is different with the text what you type in “find and replace” dialogue.
Although you know the reason leading to the problem, but it’s difficult to solve it. Because it’s not easy to change the encoding of all the text in Excel, especially when you have huge amount of text in Excel. You need to find some ways to convert the encoding of you text.