Thank you Jason, I apologize for the vague description.
I began investigating the products and found many had an empty keyword field. It was reported that searching for "eye wash" did not bring up anything, while "eyewash" gave the expected results. I added "eye wash" to the keyword field and now the search brings up the expected results.
Does the order of the search criteria (the AND, OR) affect the results? I have found the when a user searches for a safety product, both products (ProductPrice > 0) and rentals (ProductPrice = 0) appear, even though it seems they should be filtered out.
$WADbSearch1->addComparisonFromList("JoinCategory","S_ProductCategoryID","AND","=",1);
$WADbSearch1->addComparisonFromEdit("ProductName","S_ProductName","AND","Includes",0);
$WADbSearch1->addComparisonFromEdit("ProductSKU","S_ProductName","OR","Includes",0);
$WADbSearch1->addComparisonFromEdit("ProductManufacturer","S_ProductName","OR","Includes",0);
$WADbSearch1->addComparisonFromEdit("ProductKeywords","S_ProductName","OR","Includes",0);
$WADbSearch1->addComparisonFromEdit("ProductShortDesc","S_ProductName","OR","Includes",0);
$WADbSearch1->addComparison("ProductDeactivate","0","AND","=",1);
$WADbSearch1->addComparison("ProductPrice","0","AND",">",1);