It is always best to ask first rather than mess up and us needing to fix it so nothing weird in it at all and sorry it took me this long to reply, I forgot about it for a day since I hadn't had the time to reply when I first saw it.
What's best depends on who uploaded it, you can check their signature(bottom of every post) if they explain what to do since some will do that and if specified, whatever is there is best.
You can check on peoples profile the last time they logged in if they don't block that information so you can know if they are still active or not.
If their name is stricken off, that mean they are currently banned so they can't fulfill any request or see their messages(or even received any if I remember correctly).
By default, the thing to do is to ask in the thread itself for a re-upload since anyone subscribed to it will get an alert and people other than whoever first uploaded it will see it and have a chance to upload it.
Some will prefer you contact them either by pm or on their user page since they'd rather make sure to do it themselves.
If you feel it truly is too much to request in every thread or to potentially many different person, then it's also an option to use the request section, but that's usually more for things that aren't posted on the forum in the first place.
Look for the Club Forums section and there is a request subsection in both the JAV and idol subsection inside of it.
A lot of the active uploaders also hang out on IRC(details at the bottom right of my signature) so asking on there is also an option.
Just don't post the exact same request in multiple places since that would be cross-posting, which is against the rules.
If you need more details about something or have other questions, don't hesitate to ask.