You need to make sure that all cursors are closed before turning off
tracing. STAT lines, which contain the execution plan are only emitted
when the cursor is closed. If you are using sql*plus, exiting without
turning off tracing will cause the cursors to be closed. Truning off
tracing will not automatically close any of the cursors.

If you don't always see timing statistics, is it possible the query is
parallelized? If so, the information will not be written to the user
process.