I've found out what the problem is! It's in the settings, and it isn't a bug. Suppose you have set the first parameter of the plugin to "false" and the second parameter to the number of battlers there are. In that case, it won't add a turn if you try to because you have set the maximum amount of turns there, and the amount of battlers is equal to or greater than that amount as every turn you start with as many turns as there are friendly battlers.
Edit: You can increase that second plugin parameter to include new max turns if you want more turns than the number of battlers. Or you can set the first plugin parameter to "true" and make the maximum number of turns based on the number of alive battlers at the start of the turn. You can set the maximum number of turns per battler in the third parameter.