Zabbix does not execute an operation if "Type of calculation: AND" is used.
How to reproduce:
1) create two trapper items and triggers (e.g. "Trigger 1" and "Trigger 2") on the same or two different hosts
2) create an action with some operation (e.g. send email) and the following conditions:
Type of calculation: AND (i.e. A and B and (C and D))
A Maintenance status not in maintenance
B Trigger value = PROBLEM
C Trigger = Server: Trigger 1
D Trigger = Server: Trigger 2
3) put the triggers in the PROBLEM state and check result. Zabbix will not send email.