Go to your local Walmart. Buy MIMI Litter; it is the best.
You will never go back to scoopable of any kind once you try it. [ I am a Walmart Associate, so I've tried them all. ]
It is porous silica sand and is white & blue crystals. It comes in 4 lb. bags only, it is very lightweight, and with only 1 cat it is supposed to LAST A MONTH! It's just $4.17 a bag. I have 3 kitties & 2 litter boxes and I change mine out totally about every 10 days. Some Walmart's do not carry it, in fact the one I work at quit selling it a few weeks ago & now I have to go to another store to buy it. It is also sold on-line. I think you will love it like my 3 meows & I do. - I just wash out the litter boxes with Dawn and very warm water between changings & sometimes use "Scrubbing Bubbles" foaming bathroom cleaner in the litter boxes too.
If that does not do it, toss out the litter boxes and buy new ones. They may have permanently embedded bad odors in them from continuous use.