I've been using Tidy Cat (regular clay) litter for years now without real problems. We have three cats. I scoop daily and change the litter once every week--though days six and seven are pretty ripe!
About six weeks ago, the litter box became a hell-hole. We could smell it round the clock, regardless of its apparent cleanliness. One of the cats stopped going inside it, using the mat outside the litter box or just the floor in preference to the Stinky Armegeddon that the box had become! We tried scooping more frequently (which was almost impossible) and began changing the litter every 4-5 days, with no positive results.
Before purchasing a new litter box, I just decided to try a different litter--Fresh Step, which has carbon in it to absorb odor. We're using this now with absolutely no odor problems whatsoever.
The cat who had been refusing to use the litter is now happily thinking inside the box again.
Our cats' diets had not changed. None of them have or had any health problems that would contribute to the problem. We can only assume that Tidy Cats changed its litter formula, which is really too bad as it was a good product at a reasonable price.