Multiple keywords in advanced search

I have around 9000 items in the database. If I use the advanced search to search for the term oyster I get 109 results. If I search for dredge I get 16 results. 3 of the items contain both oyster and dredge but if I enter those terms in the keyword search I get 0 results. I must do doing something wrong but not sure what.

Which search are you using, the item-specific one or the sitewide search?

Thanks for getting back to me

The Item search. Putting the keywords into the "Search for Keywords" field in the admin/items/search page.

Are you using any search system?

I am using the Omeka admin/item search system as described in my earlier post

Is your site publicly available? If so, could you share links to the searches that aren't working?