+1If the mouth piece change doesn’t help, check on hose length. The hose may be pushing or pulling the second and you are biting down to hold it in place. Possibly a swivel or routine under your arm with a 40” hose and a 90degree fitting.
A small area in my lower gum really appreciate it. I even have to trim off a tiny part of the skirt in my mouth piece. All is fine now.
Surely this is not advanced scuba topic!