So in my CoS game, Strahd showed up in the second session and while escaping the party threw a fireball at them. This instakilled the wizard who was already low on hp. They were able to make a deal with Morgantha and bring him back (now owing her a soul, or the wizard's bodyguards soul is forfeit.).

But ever since then, any time Strahd shows up the party immediately books it. It's been nearly a year since that moment, and no longer are they level 3. They're level 7. In the last session we had they found the Sun Sword, and Strahd showed up not too long after. And I was hoping to move the plot along in that moment, but as soon as they realized Strahd was coming the wizard turned the whole party invisible and they booked it out of there.

I tried to hint to them after the session while we were talking about it that they're a lot higher level now, but i'm not sure if it''ll do much.  I'm hoping now that they have the sun sword they'll be a little more confident.

So far though, the campaign has been a series of loose ends and sidequests and I'm hoping to move them onto a more plot thread.

Drop the dinner invitation on them, and/or have some suitable NPC praise them for finding the weapon Strahd probably fears the most.

I think they're headed back to the abbey in Krezk once they deal with the hags (which is their next order of business) and I've introduced them to the abbot's bride.  So maybe I can drop Strahd in to visit this bride the abbot wants him to meet. (Though I worry about strahd showing up *too* much throughout the campaign). I suppose I could have him invite the abbot and Vasilka to dinner and the abbot could send her with the party

I was initially hoping Strahd would find them with the sword after he showed up so I could drop the invitation on them after that, but they just ran. I know the paladin really wants to actually talk to strahd some, so i hope that will encourage them as well. 

First, if the party is still afraid of Strahd, I consider that a big win. CoS is all about horror and the party should feel like they're being hunted. The longer you can keep that going the better. My party's level 6 and they're already starting to realize they have a fighting chance, at least in a fair fight.

That said, if you want to move the story forward in a controlled way, you have many options. Don't forget about Rahadin! He'd do anything for Strahd; you can have him approach the party instead. There's also Fiona Wachter, Madame Eva, Luvash, Arrigal, and Kiril, all of whom are allied with Strahd and can all deliver a message for him.

Or you could just keep sending Strahd after them. Thanks to his Legendary Actions, he can move faster than the party, and so can his wolves and dire wolves. He can also grapple the party very easily. Escaping him should be difficult. If he scries the party overnight, he can also bring a large number of minions to lay siege on wherever they're currently staying.

Don't be afraid to deviate from what the book says if you think it'll move the game in an interesting direction.

That's party has to go back through Vallaki to get to the hags so I might have Lady Watcher invite them over. They inadvertently allied themselves with her against the burgomaster by killing Izek, so she considers them allies. And after the whole festival debacle, I plan to have her in charge of Vallaki. (They're going in expecting it to be overrun by zombies and it's going to be quiet. too quiet)

They do know that she's allied with Strahd, so that's a good way of getting a message to them, or potentially for Strahd to find out they have the sword. 

Thank you!