Don't limit yourself on date ranges - Some of the most formative and germinal articles on important nursing theories date back several decades.
Try new places - using non-nursing and medical databases, such as psycINFO and Sociological Abstracts might help you locate theory articles for a variety of contexts.
Try out searching for specific authors of theories such as Olson AND Hanchett
Check for how often the articles have been cited using Google Scholar. The more an article has been cited, the more influential it is.
You can also be directed to relevant articles through tracing these citations in Google Scholar.