Hi! The animation is tied to the length of the conversation; basically I made it so it slides in only when there is one message, so either when the game start or after a "novel clear".
In a previous version it was tied to a variable that you would turn on and off to play the slide animation once (or else it would play for every message). It was just way too many repetition since you would need to change the variable a lot throughout the game; but ping me again later and I will give you the instruction to edit the code, so that you can play the animation when you want to (I can't at the moment, but I would be happy to help later)