Hello,
Please confirm if you have the security policies configured on site B and site A SRX firewall to allow the traffic initiated from site B to site A. if not then please configure the security policies on both the SRX accordingly.
For more information on configuring security policies on SRX to allow traffic in both the directions in policy based VPN please refer the following document:
http://www.juniper.net/techpubs/en_US/junos12.1x44/topics/example/ipsec-policy-based-vpn-configuring.html (Please refer Configuring security Polcies section)