In reply to sri205:
Since uvm_config_db set() calls are typically made in the build_phase(), there is no way of knowing what the hierarchy looks like. This makes it very difficult in determining what is set in error.
Also, you should NEVER use the default_sequence. It is very difficult to debug (as you discovered), and can cause unintended difficulties. Instead, you should start() your sequences in your test sequence.
Additionally, you should NEVER use any phase other than the run_phase(). By keeping everything in the run_phase(), you maintain compatibility with other IPs and environments.