When the message displays that it cannot connect to the subscriber, it is commonly because of the following reasons:
The subscribing server is busy
The subscribing server is down
The authentication is failing
Before correcting the problem, verify if the error message is occurring for one agent on a specific server or if it is the majority of the agents connecting to the subscriber with this problem.
If it is one subscriber, verify that the agent is not pushing a large amount of transactions. Also, verify the both the CPU and disk space as well as the database size to rule out any of these as an issue.
Next, try to ping the subscribing server to verify that a connection can be established.
If it is only one agent that is having the issue, stop and start the agent and wait for it to connect. Sometimes this needs to be done multiple times to establish a connection.
This issue may also occur if the agent has lost permissions to the destination subscriber. This seldom is the case unless it is a new subscriber.