postgres/R access problems

Hello

I have a problem that has exhausted my ingenuity and would like pointers
to a solution, or at least where to debug.

- I am using R and postgresql.
- I have databases on two different servers.
- I have a few users, who use different client machines - whose R
installations are identical (rsync'd)

From one client machine I can access databases on both servers, using

both psql and by connecting using db.connect()

From another client I can access the databases on both servers using

psql, BUT I cannot connect to the database on one of the servers using
db.connect()

It looks like an authentication/authorisation problem. There are no log
entries anywhere that suggest auth/auth are failing - not in the
postgres logs,not  in the usual /var/log/* suspects. I'm using the same
user/password for psql and db.connect(). 

Any suggestions? 

Thanks

Brendan Murray