As you know, cats have a will of their own and sometimes they need a little extra help or persuading. If you still have some, put a little of the cat litter they were used to using on top of the PeeWee pellets. Or put faeces you find outside the litter box into the tray for a while so that it takes on the cat's scent. If your cat does start to use the litter box now, it is important to gradually reduce the ratio of old litter to PeeWee pellets until you eventually only use the PeeWee pellets. Do bear in mind that as long as you are still mixing the PeeWee pellets, you will smell the litter box a little more.