Is there anyone know sp summon burst condition?

[Axess-kun]

Is there anyone know sp summon burst condition?

 

Until now, I tried a lot but still don't know the condition to active sp skill of summon burst.

Sometimes, it appear and almost always it not appear.

 

At least every 7 turns (not 100%) it'll show the sp skill of rank b (and below) of summon burst.

Tested with MP 100% of both character. Character relationship is 5 hearts (MAX) with protagonist and 3 hearts (MAX) with other character.

 

P.S. Tried to search lots of walkthrough site and not found any clue.... (including Japanese site / 2ch / etc.)

 

OTL