Monitoring Amazon RDS with pgtop
I don't know if that applies to Amazon RDS, but a possible reason for getting an empty list of queries with pgtop is when using a version older than 3.7.0 (released in August 2013) against a 9.2 or newer PostgreSQL instance.

I just noticed this with the ptop 3.6.2 package that currently ships with Ubuntu 12.04 LTS. I guess this may be also not fixed in other distributions. 

The empty list problem occurs with older versions of pgtop because they expect the postgres process ID in  pg_stat_statements.procpid, and this column was renamed to pg_stat_statements.pid in PG 9.2.

This is combined with pgtop being unhelpful by not reporting any error when its SQL command fails. Instead it just displays a blank list of processes. The problematic query can be spotted in the server's log.