Bubble in lip from filler 1 year later. I want to redo my fillers. How can I avoid it in future? (Photo)

I had Restylane lip filler over a year ago injected into my lips. I have always noticed a small lump inside my lip after but they told me it would go away or eventually dissolve.Now over a year later most of my filler has dissovled and I am still left with this small lump/bubble inside my lip. I want to redo my fillers but I am afraid to ask what the bump is caused from and how to fix it, and avoid for the future. Please advise