No-one likes wasted spells — Justification
When a creature casts a spell with the intent to hold it until something happens (See Ready, PHB193) they have their chance to cast it as normal, but if the opportunity doesn't appear the creature can use it action at the start of their next turn to maintain concentration. After that, they have the choice to cast the spell as normal (as if they just cast the spell) or ready it again using the same trigger or a new one. The Creature can move as normal during this turn.
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