At the time this goal was released the mystery fragment was a real struggle to find. there is a note to say the drop rate was doubled to give you a better chance of finding it. Like all other random drops, this one is definitely a rare drop! Are you mines L5? If they aren't, the drops will be the same, but the collection time reduces so you can fit in more collections which also increases your chance of getting the drop. Whoever the first person was who said this game requires a lot of patience, they weren't kidding!
we are willing that fragment to drop for you soon!
https://forums.storm8.com/showthread.php?72574