Helmet model not showing up on head.

Hi there, so I made a model for a gas mask and using a block like glass and placing it on the player's head works just fine, but if I make the item i.e. an iron helmet so the player can equip it simply, the model doesn't show up on the player's head.
p.s., I'm using optifine CIT, so it might be a problem with that, idk, if so, I'm okay with switching to custom model data.