Stops working after a couple months?

Hey, I first learned about not using shampoo from my brother. He told me to just use conditioner and I was very skeptical. I used just conditioner for a few months because it was working very well but it slowly stopped working. I would have less and less time between having to wash it because it was getting greasy faster. I came here to look for why it wasnt working and I found out that there is silicone in the conditioner I was using. I switched to Loreal EverPure conditioner and it started working great, I could go almost a week between washes. Now, a couple months later, it has slowly stopped working just like the other conditioner. Now, even if I wash it with conditioner it is still greasy. It's like my hair slowly built up immunity to the conditioners.

If you know whats happening to me and can help me out, that would be very much appreciated! Thanks for reading :)