Harold, you can't send a follow-up. If you *must* communicate with the client before the client messages you, then you'll have to withdraw your proposal and send a new proposal, but I think that will be a waste of connects.
Quite a lot of proposals on Upwork go unanswered forever. Don't waste your time or energy thinking about them. Put in your bid and go on with your life--those clients will either come back at some point or they won't, and there's nothing you can do to influence the outcome once you've submitted your proposal.