Site to Site Tunnel - Traffic just flowing in one direction.

Question

Greetings to everyone,
I have configured an IPSec (Site-to-Site) tunnel between an ASA5510 and a Linux Sytem connection a Network A with a Network B in the following way:
* Diagram:
                                                           #---------------IPSec-----------------#
private network (A) ---- Linux Router (GW1) -------- WAN -------- (GW2) ASA5510 ---- public network (B)
* Results:
I have checked the IPSec Tunnel on the linux Router and both Phase 1 and Phase 2 are UP. ASDM shows also an IPSec connection with the correct parameters (GW, Local Network, Left Network etc.).
If I have correctly understand it "show crypto iskmp sa", "show crypto ikev1 sa" and  "show crypto ipsec sa"  show also that the connection is correct and UP.
*Now comes the interesting thing:
If I ping from Network A to Network B, the icmp echo request go thorugh the tunnel and I can see the Rx bytes on the cisco ASA going UP.
If I ping from Network B to Network A, I do not see any Tx Bytes on the Tunnel. The Linux router does not also see any packets going through the tunnel.
When I ping from Network B to Network A, the Firewall Logs ICMP Denies. That means that the traffic from B to A, I do not know why, is not matching the corresponding Tunnel ACL, the icmp packets are being routed to the default gateway instead through the tunnel and they are then matching a less specific droping rule on the main firewall. 
*Configurations:
I have specially configured a Crypto Map that matches the Networks in Both directions.
There exists an ACL that permits the traffic in both directions.
There exist a NAT rules that permits traffic between both networks without being NATed, in order for both networks to transmit through the tunnels freely.
* Ideas ?
crypto map?
NAT?
ACL?
interface level security?
Thanks in advance.

Answer 1

Hi Guys,
Thank you all for your help. The packet was being dropped on the "implicit rule", that means that the packet was not finding an ACL to match.
I checked the ACLs that the VPN Wizard generates by itself when used to configure an IPSec connection, and the ACLs where correct and "before" the implicit rule . (They are called by default outside_cryptomap_"number")
It seems that since I am not using "sysopt connection permit-vpn" I have to add the same ACLs to the "Local Network" interface (VPN_LAN).
Since there was inbound ACLs related to the VPN_LAN interface, the firewall jumped directly to the "implicit rule".
So the result is that I have two times the same rules first inbound on the VPN_LAN and second on the default outside_cryptomap ACLs.
Greetings,
Daniel