Can only see Server 1 from Server 2 when Server 2 can see Server 1

I have a small network of W2K servers, say S1, S2 and S3.

I can ping each machine from each other.

On S2, if I go to windows explorer and type in \\S3, then I see the shares
available from S3 no problem. The same is true for S2 from S3.

However, on S2, if I go to windows explorer and type in \\S1, I get the
message 'Windows cannot find \\S1'. If I then go to S1, go to windows
explorer and type in \\S2 , then I see the shares available on S2 and I can
then access \\S1 from S2.

The same is true between S3 and S1.

All computers are members of the same domain (called 'admin' ).

Any ideas ?

Thanks

Steve