I would tkprof the trace file and focus on Wait Events section of it - you may want to include SYS=N option.

Risking to be stating the obvious please include...
alter session set max_dump_file_size = unlimited;
alter session set timed_statistics = true;
alter session set sql_trace = true;
...before actual alter session set events blah blah blah

Trace may be hitting max_dump_file_size value so it appears not to be tracing - that's the reason of the first command.

Also check you have enough space on dump file directory.