It worked fine with the other players, but not with this one, as said above it’s very inconsistent.
I’ve checked that the attachments were still there, and all of them are correctly positioned and move correctly, even the align position has Attachment0 and Attachment1.
Is there anything which causes an align position to stop working at any point on client?
This is very likely because of NetworkOwnership. If the server does not specifically say part:SetNetworkOwner(player) or part:SetNetworkOwner(nil) (server), then it will use NetworkOwnershipAuto, meaning that the person who is in charge of computing the physics of that object will change based on things like distance and other factors. If that person is not your client then the Constraint will do nothing at all (because youre not in charge of it).
Are the attachments also created on the client? What’s the full story for how it is all set up? And can you show a video of it not working and maybe it could be related to the underlying code?