For some inexplicable reason Quicktime no longer plays a lot of my avi clips, this was not an issue a year and a half ago, any idea why and how to fix this? Also the clips no longer have a thumbnail, everything was fine till recently.

Recently I am unable to play AVI videos on quicktime that I have never had any problems playing before, the thumbnails of the clips also don't display. Has there been some change to Quicktime as there was never an issue since I've had my macbook pro from 2012 till present. Note some AVIs still work, the issue is more the fact the thumbnails don't display as I do have an alternative player that can play these clips. Any suggestions but mostly solutions are appreciated!