In Python socket programming, why does recv()-ing data directly from the sockets only gives you the first message?

I'm playing around with socket programming in Python 2 and trying out select() for the server script. When I have the following code for the server:

print('Server started in port {}.'.format(self.port))
server_socket = socket.socket()
server_socket.bind((self.address, self.port))
server_socket.listen(5)

client_sockets = [server_socket]
while True:
    for s in client_sockets:
        if s is server_socket:
            client_socket, address = server_socket.accept()
            client_sockets.append(client_socket)

            print('Connection received.')
        else:
            data = s.recv(200)
            if data:
                print('Received: {}'.format(data.decode().strip()))
            else:
                client_sockets.remove(s)
                s.close()

The server only receives the first message from the client. However, the second and later messages will only be received when the client is restarted. This baffles me (of which I attribute to my inadequate knowledge in networking). The data seems to be buffered. Why does this happen?

I did try this:

client_sockets = [server_socket]
while True:
    readable, writable, errored = select.select(client_sockets, [], [])
    for s in readable:
        if s is server_socket:
...

And finally, the server can now receive the second and later messages from the client.

Here is the code for the client:

class BasicClient(object):

    def __init__(self, name, address, port):
        self.name = name
        self.address = address
        self.port = int(port)
        self.socket = socket.socket()

    def connect(self):
        self.socket.connect((self.address, self.port))
        self.socket.send(self.name)

    def send_message(self, message):
        self.socket.send(message.encode())


args = sys.argv
if len(args) != 4:
    print "Please supply a name, server address, and port."
    sys.exit()

client = BasicClient(args[1], args[2], args[3])
client.connect()
while True:
    message = raw_input('Message: ')

    # We pad the message by 200 since we only expect messages to be
    # 200 characters long.
    num_space_pads = min(200 - len(message), 200)
    message = message.ljust(num_space_pads, ' ')

    client.send_message(message)