The setup is as follows:

1. SBS 2011 in datacentre on subnet A
   
2. Domain PCs at another location on subnet B
   
3. There is a site-to-site VPN.

The domain PCs have joined the domain and have the SBS as their primary DNS server.
The domain PCs can ping the DC but the problem is that the DC cannot ping any of the remote subnet (subnet B)

SBS --Switch -- Router A ------------------- Router B -- Switch -- Domain PCs

What is strange is that router A can ping any host on the subnet B.
Another host on Subnet A can also ping any host on subnet B.
It's only the DC which cannot ping anything to that specific remote subnet B.

I did a tracert from the SBS to router B. The packet reaches Router A from the SBS but then it fails.

Am I missing some specific settings that needs to be done when SBS is on a different subnet than its member pcs ?