Okay i had this problem once in Fallout 3 and i know there was a fix for it, currently i changed some info on a hat and now it isn't appearing on the NPC's head but to the left on there shoulder.

how do you fix this again ?

other non modified hats still work properly, but none of the hats i mod.

modded Glasses don't work ethier.

Well i guess you can't modify a hat to where only a NPC can wear it Same for glasses.

Under the character tab on the top toolbar of the GECK, there is an option called 'Update FaceGen model availability'. That is what your looking for.

Gunmaster95