Help Kinda Perplexing problem: Some GIFS animate & others don't: WHY??

[bowzerbird]

I see others have this problem:
So again, can someone help me with this?
I have a few GIF files on the Samsung s5. Some will animate, others won't. What determines which will and which won't?
Is there something in the coding that permits the animation in some & not others?
On my s5, I have the latest OS and everything is up to date.
Would LOVE to solve this if anyone has a solution.
My thanks in advance,

Ellen