During load test, some clients are able to connect only after repeated attempts

I am using Executor service in the example, because it is similar to my FIX Gateway, in terms of using AcceptorTemplate

1. I have a simple client that connects to Executor service and puts a MarketDataRequest for every 10 milliseconds, total 100 market data requests.
note: 'UnsupportedMessageType' errors are expected for all MarketDataRequests

2. two clients are started for every second. total 30 clients are started

Issues:
-> connection is not established for some clients for some period
From client logs:
(Pending connection not established after 2146 ms.)
(Pending connection not established after 2850 ms.)
(Pending connection not established after 9795 ms.)

-> server is not responding to logon requests promptly for some clients
From client logs:
(Initiated logon request)
(Disconnecting: Timed out waiting for logon response)
(Pending connection not established after 4648 ms.)
(Pending connection not established after 21948 ms.)
(Initiated logon request)
(Received logon)