Can't return data from SQL/Plus and Toad but can from SQL Developer
Sounds like the network buffer is being filled but not flushed.  SQL*Net performance in this area is often managed by setting tcp.nodelay=yes in sqlnet.ora; also look at the send_buf_size and recv_buf_size parameters.

SQL*Dev working and not SQL*Plus are what pointed me to this type of troubleshooting approach.  SQL*Developer uses jdbc behind the scenes to do its fetching.

hth!