Likelihood is that they are too big, either in pixels or kilobytes. I haven't seen any yet (see mine, for instance) that didn't work if the size specs were good (which is: 120 by 120 pixels or 75.0 KB (whichever is smaller)).
I believe if you upload one that is over 75KB, it will just kill the animation. Please check the file size and if it's 75KB or less, let us know and we will research it further.