One bougainvillea has already died. The two bougainvillea next to it are starting to drop leaves and look similar to the dead plant. Right next to the dead plant are two new bushes from the same root system which are perfectly healthy.

They are not getting overwatered. Possibly under watered. It has not been cold at all this winter so no trauma.

What should I look for on the dying plant to diagnose the problem?

What tests should I perform to discover a solution?