Can't Change Custom Icons

History:

Recently I added custom artwork to some video files I have (m4v) using the following method: Control click the file, select "get info", click on the icon at the top left of the window, hit control v to paste the artwork.

Problem:

There are a few files that I wanted to change after I had added custom artwork. I deleted the original artwork and the original "preview" icon returns. However, no matter what I paste the original custom artwork always returns, not the new graphic. This affects several files, I haven't tried on others because I don't want to create more files with the wrong artwork.

Attempted solutions:

-Pasted various images in an attempt to affect change.

-I've tried deleting the custom icon and pasting a new one (It will return to the original "preview" as icon state).

-I've done a reboot and then reattempted.

-I've relaunched finder.

Specs:

OSX Yosemite, fully updated

MacBook Pro

Help! Thanks