How zabbix select items for update

[pilot]

Not all items are updating.
[no data] for many servers.
All problem items are in zabbix.items
I can't find which scripts select items for update.

Zabbix agents runned.
Sometime before all items worked well

[pilot]

Analyzing of sql logs helped me to find error.
Updating disable_until and errors_from in hosts table to 0 solved the problem.