But lately it doesn't seem that way...

I'd say it depends. Korra and Tenzin are the only two who, in my opinion, have been consistently at the foreground of each season. I think that's down to all the other characters being rotated about and introduced in varying seasons. Asami, for example, got the most focus after Korra in season one (in my own opinion, certainly in terms of personal character stuff) but has been pushed a little more to the sidelines in later series - probably because most of her personal stuff was tied up with the equalists and season one. Bumi and Kya haven't really been around this season and only Bumi made a cameo appearance in season one, but they were pretty significant in season two. In contrast, Lin was a very minor character in season two but very prominent in seasons one and three. Even Mako and Bolin haven't always had much attention paid them at varying points in the series. Jinora was only a secondary character in season one but has since risen to become a pretty major player - the list goes on.
Honestly, I kind of think we're trying to ascribe something of the Avatar formula to a very different show by labeling a group as main characters. Korra has always been more plot driven in its series than Avatar, which makes sense since they have to fit a full story into fewer episodes, and so there is less of a primary focus on the characters. And actually, I quite like it. It's a different way of doing things, and I think its very true of how life works. Some people are more significant in certain events of your life than others, you don't necessarily go through with the same tight-nit group (especially when you don't have the same continuous goal for an extended period of time). Makes sense that not everyone would be involved in the big earth shaking event if they all have their own lives to lead, and are off doing their own thing.

^Wow well said @Tsmith120

That's a good point Tenzin always seems to be there.

I think the main characters are Mako, Bo, and Korra.
The entire series is about Korra and her "growing" as an avatar. I use "growing" loosely.... but the show has her name in it. She's the main star who is in almost every scene of every episode.
Mako is Bryke's favorite, so naturally, the story revolves around him and his shoddy character arcs.
Bo is one of the main characters, and as the books have progressed, his clown-like actions and gestures have gotten him to be a central focus. And now with his new lava bending, he now is a shoe-in for being a necessity in this last book.

It's unfortunate, but I cannot call Asami a main character because she isn't treated like one. As much as Bryke says that Asami is part of the "Krew," they are full of shit.
Asami is ALWAYS cast aside and never given a prominent role, unless it is some sort of servant role that aids the avatar. Hell, in book 1, she was put in as a plot device to start the love triangle, and we saw how she was shit on repeatedly in that book. She was only useful when she could supply Mako, Bo, and Korra with money and gadgets. Then in book 2, Asami is dealing with her failing company, and we actually think she may have a significant role once again. But nope.... Asami is given one episode that turns more into an episode about Mako and his police work. And then Asami is inserted as the pawn for the rebirth of Masami fueling the fire between the ship wars. And then for the rest of the book, Asami provides transportation, is shit on again by Mako, and then isn't seen. And then in Book 3, we've got Asami actually having some lines..... but in actuality, it turns into Asami blatantly fawning all over Korra, doing all that she can to support Korra, providing transportation, saving Korra, and basically being Korra's handmaiden. It's funny though..... the Korrasami friendship is shoved in our face during this book, yet the friendship is ridiculously forced and extremely one-sided. And now Book 4..... we've seen Asami in one episode where she has said a total of 4-5 lines. And that's it, whereas, everyone else has gotten a good portion of an episode to themselves where they get to shine, or show their current struggles. But nope.... not Asami. So much for her being one of the main characters. -_____-

The fire ferrets

In my honest opinion. Tenzin was a more main character as compared to Asami.