It is important that Celica has enough Celestian energy (at least half the bar). At the same time, there should be no parasites on her body, such as a talking hat (Velvetia) and an element of underwear (Busty). Sometimes the mushroom monster does not show the captive monster if Celica has a trace of demonic influence (fixed with cookies from Margarilda). But usually the first two conditions are enough.
In any case, it is important for you to make sure that Booty is visible on the mushroom monster, and also that you know when to use Celestian magic to prevent the monster from escaping.