If you select the Also search on Skills checkbox, your scanner will search for keywords in the titles, job descriptions, and skills. The scanner will search for the keywords that are specified in the main search field, and also those that are in the Include any of these words field (if any).
If you have provided keywords in the Exclude all of these words field, then the scanner will exclude those keywords from the search results, even though you have selected the Also search on Skills checkbox. As a result, after clicking on the Also search on Skills checkbox, you may see fewer jobs in the Preview Results.
Most often this occurs when there is a large list of excluded words. We recommend that you review the excluded keywords and make sure that it does not include the skills that you want your scanner to search for. We also recommend attempting to decrease the number of keywords in the field in order to fix the problem. You must go over each keyword, remove any duplicates, and look for any syntax issues.