Reasons for treatment-resistant depression and anxiety...?

I was diagnosed almost six years ago with depression and was put on Zoloft which worked perfectly for six years. Last year however, it started to stop working and since then, I've developed Panic Disorder, GAD, and Depression (again). For the past year I've tried four other SSRIs and an SNRI but none of them helped. Could I have developed an immunity to these types of medications because I was on Zoloft for so long? If not, what else could explain the treatment-resistant depression and anxiety?