When a "pull" occurs, your client is attempting to download the file by asking for it from the other server. When you are behind a firewall, this type of connection does not work most times.

To compensate for this problem, a Push request is started which allows your computer to initiate the connection rather than having the other computer (which gets around some firewalls). This will never work if both computers are behind firewalls, as neither firewall will allow an inbound connection without an outbound to "authenticate" it.