this could be either on the proxy or server side; leaving component empty for now.

test setup : zabbix server on one system, restarted every second;
zabbix proxy on another system.
one host (monitored by the proxy), one trapper item. with sender, an incrementing value is sent to the proxy for that host every second.
proxy database reveals that it has received 42 values and claims that all 42 were successfully sent to the server.
server database actually has most of the values missing.

present values :

2013.Apr.19 14:09:24 38
2013.Apr.19 14:09:23 37
2013.Apr.19 14:09:22 36
2013.Apr.19 14:09:21 35
2013.Apr.19 14:09:20 34
2013.Apr.19 14:09:09 23
2013.Apr.19 14:09:08 22
2013.Apr.19 14:09:07 21
2013.Apr.19 14:09:06 20
2013.Apr.19 14:09:05 19
2013.Apr.19 14:08:55 8
2013.Apr.19 14:08:54 7
2013.Apr.19 14:08:53 6
2013.Apr.19 14:08:52 5
2013.Apr.19 14:08:51 4

edit : this problem seems to be on the server side, as checking proxy log at debuglevel 4 shows it sending the missing values to the server and getting back :

{"response":"success", "info":"Processed 1 Failed 0 Total 1 Seconds spent 0.000160"}

edit 2: sorry, it's proxy, actually.

25176:20130419:142651.895 In send_data_to_server() [{
"request":"history data",
...
25176:20130419:142651.895 In zbx_recv_response()
25176:20130419:142652.126 Did not receive response from host
25176:20130419:142652.126 End of zbx_recv_response():NETWORK_ERROR
25176:20130419:142652.126 End of put_data_to_server():SUCCEED
25176:20130419:142652.126 query [txnlev:1] [begin;]
25176:20130419:142652.126 In proxy_set_lastid() [proxy_history.history_lastid:51]
25176:20130419:142652.126 query [txnlev:1] [select 1 from ids where table_name='proxy_history' and field_name='history_lastid']
25176:20130419:142652.126 query [txnlev:1] [update ids set nextid=51 where table_name='proxy_history' and field_name='history_lastid']

edit 3 : problems were found both on proxy and server side - proxy ignored some error responses, server did not sync config cache before starting trappers