Welcome to EN World! It's great to hear about other folks running online games. As you might guess from my name, this is something I'm passionate about.
My blog might be handy for you.
Are the players in this game people you already know personally, or are they people you've met online specifically for this game?
In the end, you can't make someone care, but you can make the game more fun for them in which case they might be more likely to start caring. I've been lucky to have players who are generally quite enthusiastic about the game, but when I've had a player or two drift away, I've just accepted that and filled the open positions in the game.
I'd suggest you start by talking directly to the players who don't seem interested. Ask them what they think of the game. What do they like, and what don't they like? Is it about the game, or their own lives interfering? If they're just not that into D&D, well, that's okay. Not everyone is. If they like the game overall but have been bored by particular things that have come up so far, then you have something to work with!
You mention combat being slow and not the "juicy part", which raises a couple of thoughts for me. First, do you need so much combat? If that's not what you find to be the most interesting stuff in your game and it's also not running very smoothly, do less of it! Focus on exploration and interaction and not so much on combat if combat isn't fun for you and your group.
The other thought is, what are you using for combat? I personally use MapTool, and things go very smoothly in my games, but it's taken me some time to get as proficient with the program as I am today. Are you using a virtual tabletop of some sort? Or are you going with gridless combat that's done purely via narration?
Finally, are you doing this via typing or talking? My groups use Skype for audio, and I know that the game would be MUCH slower without it.
Good luck with your game, and feel free to contact me directly if you think I can help!
-Michael the OnlineDM