It is normal. In your nodes list, you will see different nodes constantly connecting/disconnecting. This is because Share is connecting to different nodes (users) and searching if they have the file(s) you are trying to download.when adding a new trigger, if the file isn't found within minutes, is that normal?.
or does it just sometimes take a while to find the file?. or is the file no longer available?.
But you are also correct: old files are more likely to no longer be available, and will not be "found". You can change your cluster keywords to try to connect to more/different nodes and increase the chance of finding a node with the files you want.
Of course, please also check your Log tab for any errors, make sure all your network settings are current/correct. If in doubt, try doing a very basic search and downloading a small, but seemingly popular file, to confirm your settings work. A short AVI or a JPEG should do the trick, but stay away from ZIPs and RARs because some of them have malware/trojans -- unless you know what you're doing.