Why is my rubber tree not growing properly anymore?

My rubber tree hasn't been growing properly since I bought it. At first I thought it wasn't getting enough light, so I put it in the sun on my balcony last summer. But then it stopped growing properly there and only got small, yellow-green-brown leaves that grew very slowly. This winter and now in spring, it was by my window the whole time, where it gets plenty of light, but it's still growing strangely, as always. I've repotted it and water it regularly (not too much, not too little). I just don't know what it wants. Can anyone help me? Thank you in advance.