Safari browsing protection extension keeps uninstalling

Hi There, 

I have installed the browsing protection extension in Safari for all accounts on my iMac (it didn't seem possible to install it and have it work for all accounts in one go).

However, after having logged off and logging back in, the extension appears to be  no longer installed and a reinstall is required.

I don't fancy having to do this for each of the accounts on my iMac everytime we log in. How can this be corrected?