Part changes position when being welded. How can I fix it?

I’m trying to weld a mesh to a r15 body but it keeps changing position and when I try to move it, it doesn’t move. I’ve never had this problem before.
https://gyazo.com/e54058e5551f75ebb06460ddfef5cb8a

Use WeldConstraints because they won’t change position when you weld it.

I always use WeldConstraints, I still don’t know how to fix it.

Could u send a better video than the one mentioned above since that video is not very clear.
Also make sure that u have used weldconstraints
Instead of welds and it would be nice if u shared your script also, so we could see what’s the errror.