Maintenance Ringdoll head won't stay on?

I'm having an absolutely baffling problem that I hope someone can help me with.

Recently I got two doll heads back from my faceup artist after letting them languish with her for almost a year. (She's an angel for putting up with my inhumanly slow response times, seriously.) The faceups look great and I couldn't be more pleased with them, but now I can't.... get the head of one of my dolls to stay on?

He's a Ring Teen (Crystal). I can get the O-ring to sit in the notches inside his head just fine, but the slightest pressure on his head makes it immediately pop out again. It's like the notches are too shallow. The baffling thing is, I didn't have this problem before I sent his head to the faceup artist! His "brother", who is also a Ring Teen, and who I bought and received at the same time, doesn't have this issue at all.

Here's a short video I took of what's happening.

Is he strung too tight?? Too loose?! I did pose his headless body to try and stretch his elastic some, but I did that with his 'brother' as well. I thought maybe the heat from sitting in a mail truck may have effected the resin somehow, but it's been 48 hours and the issue still persists. It can't possibly have to do with his faceup artist, because all she did was paint the outside of his head-- and again, his 'brother', who has a faceup from the same artist, has no problem at all. My mom took a look and said "well, you should just make those grooves deeper", but I don't have the tools or workshop space to do something like that. Would building up the edges with hot glue be enough? Why is this happening to begin with?!? I'm so confused!! SOS!!

Here's some still images of his neck if those help.

 

@Seafoam Shade I know sanding down something not visible wouldn't be too hard, but it just seems so scary!! What if I mess up?? What if I EXPLODE AND DIE. (This won't happen.) The superglue and baking soda trick is very useful to know about, though-- maybe I can use that to build up the edges of the grooves! That sounds like it would be doable to me, haha. No explosions.

@jessholy Sadly no, I don't... It wouldn't be too hard for me to pick some up but I worry about the cost for something I'll only use once and then let languish in my closet forever. I didn't consider contacting Ringdoll since it's been a year since I got the doll from them, but that's good advice! Thanks!

@skumring @amandabuns ..............................turning the O-ring the opposite direction worked. What the heck?! If I wrangle his head around the ring still snaps back out, but it's much more secure now. This way I won't worry about some sort of Looney Tunes style beheading accident until I get the time to do a more permanent fix, whether that's a full restringing or a mod inside his head (or both!). Thanks so much!!!