Hi,
If you are seeing missing results, then make sure that the search in terms is also enabled – that will make sure to search tags as well. Another thing might be the number of results. If there are lot of results matching, you might want to increase the results limit as well – maybe the results would be there but reaching the limit and never shown.
You can change the autocomplete and keyword suggestion sources here and here. You can choose different sources from the default “Google Keywords”.