Junk filter not working

You can send messages to your junk folder, but iOS mail does not have any spam filtering built in. All you can do is see if your email hosting service offers server-side spam filtering and enable that on your account. iOS Mail lets you move messages to your junk mail folder because, if that is the spam mail folder on your hosted accounts server, most email services have software that "learns" to filter based on what is in that folder.

If if you've enabled sever-side spam filtering on your account, then make sure that you have iOS mail set up to use the junk mail folder on the email server so those flagged messages end up in the server for use by its filtering tools.