Strange access problem! - please help!

Hi There

I have a strange access problem that I can not solve.
I would REALLY appreciate if you could please help me solve this!!

Basically, the network is a double NAT configuration as seen in the diagram below:

(64.64.64.64)WAN<-[DSL ROUTER]->LAN(10.10.10.10)<->(192.168.1.1)WAN<-[ROUTER]->LAN(172.31.10.10)<->(172.31.10.20:8000)


[R1] => WAN<-[DSL ROUTER]->LAN
[R2] => WAN<-[ROUTER]->LAN

[IP X] => 74.74.74.74
[IP A] => 64.64.64.64
A & X are external IP's retrieved from ISP
A(64.64.64.64) is an external IP that DSL R1 gets from my ISP for my network.
X(74.74.74.74) is an IP that my wireless laptop gets from a near neighbour ISP

=> 10.10.10.10
[C] => 192.168.1.1
[D] => 172.31.10.10
[E] => 172.31.10.20:8000 (Target NATed Server)
B,C,D,E are all static NAT IP's in my local network.

Both R1 & R2 routers are configured with port forwarding rule to allow access to NATed server 'E'.
R1 forwards traffic to 'C' and R2 forwards traffic to 'E'

The thing is that I CAN access server 'E' only by connecting from 'D' (pointing to http://172.31.10.20:8000)
(which is obvious as they both on the same subnet) or when I connect through my laptop from the external IP 'X'
(pointing to http://64.64.64.64:8000)

Strangely however, I can *NOT* connect to server 'E' from points 'B'/'C' from my own lan!!
(using either http://172.31.10.20:8000 (or http://64.64.64.64:8000)

I can connect from 'B' to R2 router and enter it's config gui (R2 even shows server 'E' MAC address)
(proves that my 'port forwarding is 100% OK), yet still I can only access 'E' from a pc on the same subnet ('D') or from a laptop on totally external IP 'X'

How come it manages to connect from an outside ip, yet not from my own lan ?!? How can I solve this strange problem?

Any help would be much appreciated!!

Thanks!