Can no longer open encrypted files in my Sophos app on my iPhone??

I use Sophos on my I phone and everything stored in my Sophos app is encrypted. It is set to use my fingerprint to open and log in and that works but now when I go to open my files it prompts me for a passphrase it hasn't done that before why is it doing it now? Do I need to change settings? I'm not sure what the passphrase is.