No search results could arise from multiple situations.
Conflicting filters - Sometimes there are conflicting filters (e.g. person must live in the US and must live in Japan).
Keywords are too long - Be careful of adding super long criteria or accidentally pasting in a list of keywords (we parse them if we can) as we treat each filter as a quoted string.
Too many must have filters - Try converting a few filters to Should have filters by clicking on the Advanced Search button at the top of the search page.