File transfer problem on MSN Messenger

Pls help me as this is a strange problem that no one seems to know how to resolve.

I have 2 computers (Primary A and Secondary B) networked to a hub and connected to the net on broadband.

Computer A is the main computer, connected directly to the net and giving computer B access to the net with INTERNET CONNECTION SHARING.

Here’s the confusing part.

Computer A can send/receive files to/from computer B via MSN MESSENGER. Computer B can send/receive files to other MSN MESSENGER users, however, computer A cannot send/receive files from other MSN MESSENGER users.

Why is that?