Why are flowers collapsing on zantedeschia ?

Depends which one you've got though - most of them don't like dry soil, but only one relishes wet and grows in water.

As Bamboo says, they can be a little demanding, but with watering, the key is often a good watering and then allowing something to almost dry out. More plants are killed through overwatering than any other single cause.
I'm sitting here in 30C, looking out at the patio, where I can see two small yellow Zantesdeschia in five inch pots. They are both flourishing with good flowers, and get watered once a day (and they dry out completely by the end of the day.) If the weather was wet and cool, I'd only water them when they really need it.