Scapy: sniff(filter='icmp', iface='ppp0', prn=icmp_callback)

[Veek. M]

#!/usr/bin/python

import logging
logging.getLogger("scapy.runtime").setLevel(logging.ERROR)
from scapy.all import TCP, IP, ICMP, sniff

def ip_callback(pkt):
    print '--------------------------- IP--------------------------'
    pkt.show()
    print 'IP', pkt.src, pkt.sport, '--->', pkt.dst, pkt.dport

def icmp_callback(pkt):
    print '--------------------------- ICMP --------------------------'
    print pkt.show()

def tcp_callback(pkt):
    pkt.show()
    print 'TCP', pkt.src, pkt.sport, '--->', pkt.dst, pkt.dport

def udp_callback(pkt):
    pkt.show()
    print 'UDP', pkt.src, pkt.sport, '--->', pkt.dst, pkt.dport

print 'hello'
sniff(filter='icmp', iface='ppp0', prn=icmp_callback)

I get:
deathstar> python myscapy.py 
hello

and nothing further when ping -c 2 8.8.8.8 runs - why??
Sometimes though it works (once in a blue moon) and i get the show 
output.