Function delta() on Trigger

[themons]

hi,
I want to use the function delta on an zabbix trapperd item (every 60s)

funtion : {toto:item.delta(180)}>10

the log of zabbix server returne this

018083:20060712:165401 Evaluation of expression [{11932}>10] failed [Unable to get value for functionid [11932]]
018083:20060712:165401 Expression [{11932}>10] cannot be evaluated [Unable to get value for functionid [11932]]

is anybody have an idea to solve my problem

[themons]

I try ton creat a triger likey this:

({HOST:ITEM.max(180)}-{HOST:ITEM.min(180)})>10

and the front end reply

[ERROR:Cannot add trigger]

Incorrect trigger expression '(0-0)>10'

Last info the data is only collected between 7h00 -> 22h00