when i got my cat i had the luxury of being able to train it to use the toilet bowl instead of thee litter box (shes even trained to lift up the part that humans sit on), and shes been using it for over 1 year now! yesterday and today i keep hearing her anxiously meow as though she cant find a litter box, and shes twice (be4 i borrowed the box and litter) did her business on my floor! whats going on?

They are 2 year old long haired chihuahas. They normally go to the door when they need to go to the bathroom so Ill let them outside to do their business. But every now and then they STILL go to the bathroom in my house and I never catch them in the act so I cant punish them. After 2 freakin years they shouldnt be going in the house any more. What else can I do considering they are not puppies any more and may be harder to train?
Both my dogs are male and they have been fixed, over a year ago.