Won't cover poop!

I have two male brother kittens 10 months old and an adopted female 3 weeks younger than them. I've had the brothers since they were 6 weeks old and the female since she was 11 weeks old.
The boys cover their poop but she doesn't. I get so ticked off because she's so smart I know she can do.

One of the males like to go overboard when covering his poop. He will try to dig ti the center of the earth. Both males will cover hers when they have to go again.

I tried everything I know bribing, begging, pleading, threatening(lightly) with the female but she won't do it. She acts as though she doesn't understand but she will scratch the insides of the litter box instead of using the actual litter to cover her poop.

I tried to teach her by using her paws to cover it and praise her and give treats but I only succeeded in her expecting me to give her treats for ME helping her cover her poop. She won't do it herself. 

She used to cover her poop when I first got her but I used to keep her separated from the males because she was so tiny but now she as big as they are. How can I get her to cover her poop?

Funny, my 3 yr old Female does not cover poop, but deeply covers her pee. My 2 yr old male deeply covers his poop, but does not cover his pee.

With our cats, our male doesn't cover his poo in the litter box but the female does. She will even cover his when it stinks really bad lol. The thing that gets us the most is that if he goes outside he covers it up. We've just given up on it and pretty much just scoop it out right away.

That is exactly what Maia does. She is a lazy brat. Foley covers it instead. And he also overdoes his burying spreading litter all over the floor.

She is not lazy.  She is displaying a sign of an "alpha" (top cat) personality.  It is common for alphas to not cover their poo as a sign of their dominance to the others.  No amount of pleading or begging will make her cover it, either, so you're wasting your time.  Cats are not dogs.  They don't aim to please us.  They have their own minds and personalities.  My suggestion is to get a self cleaning litter box or just resign yourself to cleaning it up yourself if it bothers you that much after she goes.