From what I can see if you are behind you will have an increased chance of getting one.
https://us.battle.net/support/en/article/000336680
For any characters that fall behind on Spark creation, extra Splintered Spark of Shadowflame has a higher chance to drop from various sources until the character is caught up.
If you don’t believe this is working correctly, I"d agree with Sendryn, and recommend submitting a bug report so our QA team can take a look.