I typically use Tools => Search in order to search across all databases.
I do not believe this search method allows ‘Prefix While Typing’. Nonetheless, at times I can enter the first few characters of a search term, hit return, and the search will find all relevant items. Other times I have to enter the entire search term to produce a result.
Can someone help me understand why this occurs?
There are many parameters that can be adjusted in the Search dialog. From your description, I’d guess that you sometimes have the “fuzzy” parameter active, and sometimes you have it inactive. For example, if fuzzy is active then you might find results by entering just “hipp”, but if it is inactive, you would need to enter “hippopotamus”. What the fuzzy option does is tell Search to find matches based on partial matches, while without fuzzy Search uses complete matches.
I usually leave fuzzy active when searching databases unless there is a single, specific result I want and I’m completely certain about spelling.