need help with a hat on a NPC (i know this isnt a script but i need help)?

im making a batman game and i have the batman model i made (r15 and i added an idle script so the NPC is breathing) and whenever i made the model of the mask and put it on him it just pops off, and when i anchor it it just stays there and moves the NPC, i need help (i know this isnt a script but this is only helpful place i know)