VPN Connects, but can't see past next router

Hello experts, and thank you in advance for your support.

My problem:
I have a Cisco 2801 router/firewall setup as our main internet/VPN
router. We have multiple VPN clients that are able to connect to the
VPN with no problems whatsoever. Once connected, they can ping and
access any devices on the network they are connected to. They are
also able to ping the IP addresses of the routers at our second and
third locations (located over Frame Relay/MPLS connections). However,
they are unable to ping or connect to any devices on the other side of
those routers. Although, if, when connected to the VPN, one takes
control of a desktop computer using RDP (Remote Desktop Protocol),
they can then access everything on all networks just fine.

In other words, we have an MPLS network setup for intranet traffic,
and all of these MPLS routers are reachable when connected to the VPN,
but nothing on the other side of these routers are reachable. I
thought it might have been the configs of the MPLS routers, however we
just switched from Frame Relay (we were having this problem then, too)
to MPLS, with new routers, and completely different configs (AT&T
managed), and the problems still persist. I think it has to do with
the 2801 router....

Any ideas? Thank you so much!!!!!