Routing 3 networks together….

Situation:
We have 3 networks: a main office, and 2 branch offices (a&b).
Main office is connected to branch A via dedicated T1 between serial interfaces, and connected to branch B via a VPN link over the internet.

My issue is routing between the branch offices. main office can communicate with both branches and both branches can communicate with the main. The two branches are unable to communicate with each other at all.

Routing seems to be set ok, i am wondering if maybe i need to be looking in the access lists?

What information would you all need to help me get back on the right track?

Thanks for all the help!

BTW main office is running a cisco 1841, office A running a cisco 2811, and office B another 1841.