This is happening due to the stop list in robohelp project settings. These words like if, of , about etc are present in the default list. Whenever search is done with option "show results that include all search words", if any word is present in stop list then no search results are shown.
And that is the reason why you are not able to search.
You have two options either to remove these words from the searched string or from the stop list.
Stop list is present at File>>Project Settings>>advanced>>stop list.
In stop list you can add the words which you don't want to be searched in the final output.
Hope it helps.
Thanks,
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