problem with PostgreSQL and LAN

Hi all,
my problem is simple but very frustrating,

I wrote some program under Windows 95 (MS VC++ 4.0).
This program uses MFC ODBC classes (CRecordset and CDatabase) to
cooperate with my PostgreSQL 6.3 (installed on RedHat 5.1 linux) via
psqlodbc driver.
I can connect to my databse, I can scroll, update, insert or delete records.
Everything's fine.
Problem starts when more than one user has access to the database.

Example:

Person A runs my program, connects to PostgreSQL database and opens
tables. Everything works fine.
Person B runs my program,connects to PostgreSQL database and opens
tables. Also everything works fine.
Both persons can scroll records and read their contents without any problem.

Now, let's say, Person A wants to make some modifications (or even add new
record)
to some records in some table. My program's execution stops and waits until
Person B exits application (!!!!!!).

ps ax command on my Linux server shows that there are two postgres processes
running with
some command line options (it's OK since there're 2 persons logged in).
And of course, there's also postmaster process running (with -i command line
option specified).
What's the reason of such a strange behavior? It seems, that postgres locks
entire database
and makes any updates impossible. Could any1 help me with that?
I'd appreciate any help...

yacol