I have no experience with this but my initial thinking is that you might have a setting that needs to be set correctly so you might want to double check all the settings and options. The reason I feel like this might be the case is because I would think most terms would provide at least some type of result. But since the Shopping system is completely separate from the regular search results, maybe they are tighter with the search results and require some degree of relevancy before they feel justified in charging the advertiser for that impression or click. With the standard organic google search results, they're not hurting anyone monetarily by providing completely irrelevant results.