I think when you are used to a medium/ larger breed whom can be housetrained very quickly, it is easy to look at a small breed and think that they or you are doing something wrong, but as already said, some never are housetrained, others can take a year, there is a world of difference between these breeds, I think it is an exception to have many of the small breeds trained by 6 months.
I know someone with a Bichon, (terribly hard breed to house train) who crates her girl on a night, and had a cat flap fitted, which actually seemed to work very well, it may also be worth a try.
But, I guess patience is the key here, taking her out very often will help you to elimate a lot of her toileting indoors, as for her she'll get it, but boy it seems to take a while.