My cat box ALWAYS smells!?

I have 2 kittens that I rescued. They are bout 10 weeks old now. Both males. we clean their litter box daily. And empty it weekly. they eat dry food only. But it seems like we can NEVER get rid of their catbox smell. I even went and got them a covered box with a door and it hasnt even touched the smell at all. Ive tried adding baking soda as well as sprinkled some baby powder or carpet fresh powder. Just enough to slightlu soften the smell. But NOTHING works! We have another cat who is 6yrs old inside and she has her own box. We rarely smell hers. They all eat the same food. We only have to scoop hers every 2-3days and change it every couple weeks just for cleanliness. Not for smell. What is going on with these 2 little ones?! Is it cause they r males maybe? I dont want to have to get rid of them. My kids have fallen in love with em. And they are super sweet!