If your search does not find enough items, the following techniques will help broaden your search:
Check your spelling
Have you mistyped or misspelt any terms? Have you considered variations in spelling of terms?
For example: Rather than searching on globalisation (Australian), also search on globalization (U.S.).
Wildcards can help with this. See 2.3.6 Truncation and wildcards.
Are you being too specific?
Start with your main concepts. Can you broaden your search terms?
For example: Rather than searching on casinos, search on gambling.
Try to think of alternative search terms.
Drop any unimportant concepts from your search strategy.
Remember, truncation is a search strategy that uses a symbol to find records that include any term starting with the word stem. See 2.3.6 Truncation and wildcards.
Could it be that not much has been written on the topic?
It is possible limited research has been conducted in an area. Before you can make this conclusion however, you need to make sure you have searched a number of different search tools. You may need to search the full-text of an article.