By default, V only searches the file being viewed for the search string. In particular, doing a Find Next will fail to find anything if no further matches of the search string can be found.
However, doing a Find Next using Alt+A (or Alt+F3), will cause V to continue searching the "next file" for further matches. If it finds any, it will load the new file and display the next match.
What is the "next file"?
You can always tell what the "next file" will be by placing your mouse over the Next File icon in the toolbar (the icon with the small right-arrow). Alternatively, you can click on the File List icon (small down-arrow) to display an ordered list of the files.
If you viewed the current file by selecting it from the V directory listing, the next file will be the next file in the directory listing.
If you launched V from the command line with more than one file (eg, V *.txt), the file list contains all the files specified on the command line.
You cannot search backwards across files.