I am with Itallian Greyhound rescue and I have to say, IGs are hard to housetrain. I believe most toy dogs are and that it's the price to pay for darling little dogs.
Probably 80 percent of IG rescues are turned in due to housetraining issues. When I get a male foster, the first thing I do is put a belly band on him. This way my house doesn't get marked, and I am not frantically chasing a dog around trying to watch him every second, although I do watch him closely. I also ignore mistakes in the house. I praise, praise, praise and praise some more when they potty outside. I also watch them when I let them out, everytime so I can praise them.
They do eventually get it, but it might take time. Don't trust too early.