OnTriggerEnter does not work on placed terrain trees

Hi,

So why does OnTriggerEnter not work for trees placed via the terrain tools in Unity?

I've a bush that I have placed and added a sphere collider which triggers a sound when the player walks through, this works perfectly when I add the bush manually to the scene; however, when I place the bush using the Unity place trees tool it only acts like a collider (impassable) and does not act as a trigger should (passable)?

Is there a specific reason anyone knows as to why this happens and what the workaround might be?

The other way I could do it is check the distance from player to the bush and once below a set distance trigger the sound but this would be very heavy on the computations due to the number of objects.

Thanks for any help,

Stephen.