Cat Box Suddenly Stinks!

I have two neutered cats, a male and a female. The male is about 10 and the female about 7. I've always been careful about keeping their litter changed and clean so the smell has been minimal. Just in the last week the cat boxes have started to really reek! I bought new boxes and cleaned the floor and 24 hours later they were reeking again. There has been no change in their diets.

Could this be a sign that one of them is ill? They are both acting normal. I'm not even sure if it's one or both that are causing this horrible smell. Any advice would be appreciated.

How often do you scoup their boxes?

I scoop every day and change twice a week.

I wonder if their anal glands are ok?

I'm not sure. They are both acting normally.

Is there any leakage coming from their rear or are their stools runny?.

If so do a search here on anal glands to see if it's the same

I can't see anything that looks wrong. I'll check out the site. Thanks.