
Firstly, have you had her vet checked to rule out any medical problems??
If she is found to be in good health, all you can really do is take her right back to basics as if she is a baby and
take her outside every time she has eaten, drank, played etc, and about every 30 mins or so otherwise. Wait with her until she performs and then praise lavishly. Repeat for as ofeten as necessary.
Also make sure you are effectively cleaning up all traces of any accidents indoors or the lingering smell will encourage her to keep returning to the same spot. A solution of warm water and biological washing powder is probably your best bet for this.
It would seem that some Yorkies can be difficult to housetrain. My friend's little bitch wasn't clean in the house until she was over a year old

Like with all training, perseverance and consistency are the key. Good luck :)