blocking

Hello,

I've a JDBC server that connects to PG using read commited connection.
This works well and I can run several serveur simultaneously.
When I run PGAdminIII I ca alse connect and browse my database. I can
update, delete data. When I try to alter the schema (ex: alter table add
column), the alter query is blocked until the servers are shut down ?

Is this a bad connection parameter in PGAdminIII or in my server
configuration ? Can you help me solving this problem ?

Thanks a lot.