Why wouldn't a filler dissolve with hyaluronidase?

I had Restylane injected in my lips 6months ago and recently it started to lump up and forms white lumps under the skin. I had the filler injected twice with hyaluronidase but it doesn’t dissolve (the doctor targeted the lumps). I went to another doctor and he had to make small incisions and squeezed out the filler. He showed me substance and said it’s definitely filler not silicone but he is not sure why it doesn’t dissolve with hyaluronidase. Does anyone has explanation for this?