How to tune "redo log space request"?

I've got a problem with an Oracle instance. The instance has a "redo log space request" (and a consequent "redo log space wait time") every time there is a redo log file switch.

The redo log files are huge (32MB) and are many (12 redo log groups of 1

file each). They are on 2 different fast disks/controllers/channels. The log buffer is set to 262144.

As I know there should not be "redo log space requests". Isn't it? How can I avoid them?
How can I find theyr cause?

TIA

                                                   
 Luca Minudel                    software designer
 Italy                             Conegliano (TV)
 e-mail                      luca.minudel_at_nline.it