I have a huge Problem with my Avatar Pls Help / Head Dissapears out of nowhere

Hey Guys, i m creating Avatars for VRC since i startet tp play. But for the last 5 Weeks i ve had some Problems. The Avatar i made has a few outfits and some sword/Weapon stuff and everytime i want to upload it i test it in the Unity-Playspace. and everything looks fine but when i upload it my Head suddently disappears. The wierd thing is every Objekt below or above is completly fine, my Hair, my crown or every hat piece from the outfits work Fine. Only the Head dissappears and my Body overall starts bugging a bit above the ground. Can anyone help me or tell me what the cause is Please…

Bounding boxes? Personally I make all the bounding boxes on my avatars the same, so instead of parts getting culled it’s the whole thing

I will check that, thanks!