I find first chapters come in two sorts -- either they come quickly and easily, or...not. I often have the most trouble with first chapters when the book is a sequel. The pacing can be nightmare as I struggle to introduce the characters and get the story going.
Why's that? Don't I already know the characters, or at least one of the main characters? Surely that has to make it easier.
Well, it means I know something about what makes at least the hero or heroine tick.
Unfortunately, that also means I know too much. There's a whole backstory from the previous book(s) and I'm as tempted as the next person to get all that history in. But if I do include it, well, let's be frank. It can be as boring as listening to the most boring travelogue. Yet I do need some of that history. This story and the characters aren't coming out of nowhere.
So the challenge becomes, how much do I put in about what happened in the previous book(s) before it gets unnecessary and worst of all, boring?
I wish there was a formula, or some hard and fast "rule," but there isn't. I have to go purely on instinct, by what "feels right." One things for sure, though -- I want the story to move, to be exciting and engaging. If I'm getting bored, I can be darn sure the reader will be.
It's hard cutting whole paragraphs and scenes. It never gets any easier. But if it's cut or bore the reader, well, out it goes. With sequels, I usually have to cut more out of the first drafts and revise and rewrite and revise again until I've achieved a balance between material relating what happened in the previous book(s) and the action of the new story.
It's not easy, but hey! What part of writing a book is?