Site to Site VPN

We have 2 sites for a client, all of the servers are hosted at Site1 and we have a site to site IPSEC VPN between netgear firewalls at each site. Occasionally at Site2, users won't be able to reach anything from Site1's network. This can normally be fixed by running ipconfig /flushdns or /registerdns. When running an NSLOOKUP at Site2 to servers at Site1, the addresses resolve correctly, but ping, web, and share access are unavailable. We've tried turning off netbios on the machines at Site2 and on the VPN, which made no difference. Adding the servers into the machines hosts file fixed the issue but isn't a permanent fix as all of the machines are laptops and frequently go offsite. The two sites are using different IP ranges, the IP address setup is as follows: Site1 has 2 public IP addresses, one for the hosted fiber line, one for the firewall. The firewall is open to WAN and has a private subnet of 125.1.X.X. Site2 has 1 public IP address to the modem. The modem then gives out a 192.168.0.X address to the firewall, which has it's own private IP range for the rest of the network of 192.168.10.X. The firewall is in the modem's DMZ. If you need anymore details please let me know.