Does v$sql_monitor show all queries?
SQL Monitoring will be triggered if one of the following conditions is present:


query is taking more than 5 seconds of CPU time
or
query is taking more than 5 seconds waiting on I/O
or
query is running in parallel
or
/*+ MONITOR */ hint was present in the query


In other words, if your query is a very fast one, doesn't run in parallel and doesn't have MONITOR hint, it won't be monitored.