Why do my lip fillers migrate to the middle of my top lip?

I had lip fillers (either Juvaderm or Resatylane, not sure which) a couple of weeks ago and all the product settled in the middle of my top lip, there were also somel lumps. I went back to my injector and had them massaged out which looked much better, a couple of days after I went again after feeling a few lumps again. This is still happening and It looks to be settling in the middle. Should I massage them? Will it always do this and how can I avoid for next time I get fillers?