Zabbix Proxy 1.8.x drop to send some data. It depends on transaction delay to DB.

In Zabbix Proxy...
Process A: in DCsync_history() calls transacation with DCmass_proxy_add_history() and DCmass_proxy_update_items().
But DCmass_proxy_update_items() is delayed for some reason.
Process B: my call DCmass_proxy_add_history() and commited. But Process A is not completed.
Process C: try to send data to Server. Then it reads the data from DB which commited by Process B. But currently Process A is not completed, so it can not read Process A's data.
Aftere then, Process A is completed. But as lastid is updated, Process A's data is never sent to Server.

So, I think that for resolving this problem, you shuld insert the WRITE Table lock for DCmass_proxy_add_history() and DCmass_proxy_update_items().