Callback fuction is not getting called

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Hello I am new to ROS. I want to send messages in the form of string from Node1->Node2 and Node2->Node1. As Node1 can publish and Node2 can subscribe to the messages. Node2 can publish the message but in Node1 callback function is not getting called.

Program for Node1(talker)

#!/usr/bin/env python
import rospy
from std_msgs.msg import String
from number_generation import give_prime, secret_no
from modulo_operations import calculate

def callback2(data):
    rospy.loginfo('I heard %s', data.data)

def talker():
    pub = rospy.Publisher('chatter1', String, queue_size=1)
    pub1 = rospy.Publisher('chatter', String, queue_size=1)
    rospy.init_node('talker', anonymous=True)
    P=str(give_prime())
    q=str(give_prime())
    secret = secret_no()
    Server = str(calculate(int(P), int(q), int(secret)))
    rospy.loginfo(P)
    rospy.loginfo(q)
    pub.publish(P)
    pub1.publish(q)
    rospy.Subscriber('c', String, callback2)
    rospy.spin()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

Program for Node2(listener)

#!/usr/bin/env python
import rospy
from std_msgs.msg import String
from number_generation import secret_no
from modulo_operations import calculate

P='asdasd'
q='ygyg'
secret = 2
client = 13

def callback(data):
       global P
       P=int(data.data)
       rospy.loginfo(rospy.get_caller_id() + 'I heard %s', P)


def callback1(data):
        global q
        global secret
        global client 
        q=int(data.data)
        rospy.loginfo(rospy.get_caller_id() + 'I heard %s', q)
        sum()   

def sum():  
        global secret
        secret = secret_no()
        client = str(calculate(int(P), int(q), int(secret)))
        print 'calculation', client
        pub2.publish(client)

if __name__ == '__main__':
        rospy.init_node('listener', anonymous=True)
        pub2 = rospy.Publisher('c',String,queue_size=1)
        rospy.Subscriber('chatter', String, callback)
        rospy.Subscriber('chatter1', String, callback1)

        rospy.spin()