Vertical Labret

So I’ve got my vertical labret pierced (I’ll insert a picture), and obviously to start off with I had it pierced with a swell bar of 10mm which was massive and you could see the end of the barbell.

I then changed it once it healed to an 8mm and it sat perfectly and you couldn’t see the bottom on the barbell, only the two balls. A couple weeks later, I could see the bottom of the barbell and it wasnt sitting perfect anymore.

I then went down to a 6mm and the same thing happened again.

Any ideas as to why?

(Btw had my lip done for 7 months and its fully healed so it can’t be swollen)