Checkpoint not complete in alert log file

 Hi,
I'm seeing "Private strand flush not complete" and "Checkpoint not complete" in the Oracle 11g database alert.log file. As my understanding "Private strand flush not complete" is similar to the “checkpoint not complete” message and can be also handled that way! Should I consider increase the size of the redo logs or should add more of them? Thank you,
> Check the redo log size
>
> SELECT group# "GROUP",
> bytes "SIZE",
> status "STATUS"
> FROM v$log;

Result:

GROUP SIZE STATUS
---------- ---------- ----------------
1 31457280 INACTIVE
2 31457280 CURRENT
3 31457280 INACTIVE
4 31457280 INACTIVE


> SELECT ROUND(AVG(1440 * (b.first_time - a.first_time)), 0) "Log switch
> time - minutes"