'Before Eval' state application occasionally not working?

So, I'm using Yanfly's Instant Skills, Buffs&States, and Skill Core plugins.

http://yanfly.moe/2015/10/13/yep-8-skill-core/
http://yanfly.moe/2015/10/30/yep-22-instant-cast-2/
http://yanfly.moe/2015/12/25/yep-50-buffs-states-core/

My Instant Cast skills are all assigned to a single skill type. They all also have:

<Before Eval>
user.addState(13);
</Before Eval>

- in their note boxes. State 13 seals that skill type, lasts 1 turn, and is 'Turn End'. So you can only use one instant skill per turn.

This works, like, 95% of the time. And then, occasionally, it just doesn't work for reasons I cannot comprehend. There doesn't seem to be any rhyme or reason that I can see. No one has any state resist traits. Every character and enemy in the game has exactly 10 Luck.

Any ideas what might be happening? Thoughts on alternative methods that might be less prone to malfunction?