3 networks, cannot use windows files

[econet]

Hello

I have urgent problem.
We added new subnet.
Now we have
Network A - to the Internet
Network B - our default local network
Network C - new local network

From B & C user can access to Internet. They also can print, see files in both networks.
But problem is that coping big files will give error (like disk error etc) - this will happen only if they use files from other local network.

Checkpoint is blocking something, but where I should search. In logs I do not see anything, also I have rules from B to C ANY TO ANY, same rule for C to B.