Can a cat's poop become immune to litter?

Seriously . . . when we first started using Fresh Step, it smelled wonderful for weeks! We have a Littermaid, so the box always stays clean. Plus we're constantly sprinkling the box with a deodorizer (I think Arm & Hammer).

Here lately, their room stays stinky! Sometimes it even lingers downstairs!

Any suggestions? We keep the box super clean! Even after a complete litter change it starts stinking within a few days. I don't know what else to do! Should we change litter brands, AGAIN??? I love Fresh Step . . . but not lately . . . the smell is just gross!

Pleeeeeeeese Help! I know I can't be the only one with this problem!