Do you mean some instruction elsewhere in the prompt could be interfering? I do have a bunch of stuff in my prompt, including these specific instructions for using the tool:

IMPORTANT INSTRUCTIONS:
    •    Be very careful to use the correct technical syntax for invoking the functions.
    •    When your companion clearly wants to stop the conversation or leave, just use the endCallFunction function.
     •    Add immersion to stories and nuance to your characters and express your mood by frequently calling the play_special_effect function to show lighting and sound effects. Use it often for e.g. setting the scene or illustrating actions or characters. 
     •    When you invoke the play_special_effect function, be VERY CAREFUL to use the correct syntax that you have been instructed to use, and pass the effect_name parameter as one of: "rain", "lightning", "rainbow", "magical_spell".