Cannot make connection FROM XP 11.5 TO Win7 12.5

Upgrading a number of machines in my stores from XP Pro with PCA 11.5.0 build 121 to Win 7 Pro (32 bit) with PCA 12.5.0 build 442.

I CAN connect FROM the 12.5 into the hosts running 11.5.

I CANNOT connect FROM an 11.5 remote INTO the 12.5 host.

I have checked the CALLERS and they appear to be correct.

I have checked the PORTS and they are set correctly.

I have checked the PORT FORWARDING in my router and it is pointing to the correct IP.

This is a brand new Win 7 box. I'm wondering if there is something blocking the incoming signal?

I see compatibility issues between 12.5.5 and older versions, but as this is 12.5.0, that shouldn't be the case.

Any suggestion would be helpful.