Your mother may be suffering from what we call 'treatment resistant depression'. So, she may be resistant to usual anti-depressants and that's probably why the doctor's trials are not working. In such cases, there are some other modes of treatment that might help. Some of them are psychotherapy
hormones supplementation, mood stabilizer
drugs, etc. You can discuss these options with the doctor. Consulting another expert might not be necessary.
Hope this helps.
Abhijeet Deshmukh, MD