Conditional formatting double space intermittently working

I have 2010

I have conditional formatting that looks for special characters that are listed on another sheet and fills color so the data entry person knows there are special characters that aren't allowed. We also look for double spaces.

Conditional formatting formula:

=SUM(--(ISNUMBER(FIND('Special Characters'!$B$2:$B$168,A2))))

It catches some double spacing where ever it exists in a paragraph. Other times in other cells it doesn't. If I just enter a double space into a cell where it DIDN'T catch it in a paragraph, it catches it.

Thanks!