ORA-1555 - 12.2.0.1 Exa

We are getting ORA-1555 on one of our daily extracts and I need to explain to the business why it is happening.

ERROR at line 1:
ORA-01555: snapshot too old: rollback segment number 92 with name "_SYSSMU92_3807070151$" too small

ORA-06512: at line 43
ORA-06512: at "SYS.DBMS_SQL", line 1726
ORA-06512: at line 24

I have an undo retention of 3 hours (query fails before that generally) I have an undo tablespace that can grow to 400G and currently only at 120G

so it does not appear to be the usual suspects so I am thinking of setting an event to capture more information to explain it better

would below give me more information why this is happening or is there a better solution?

*alter system set events '1555 trace name errorstack level 3';*

My guess is that the same blocks are simply changed too many times during the time of query so the rollback record required for CR is simply no longer available.

Jack van Zanen