Perc 6i won't set to Write Back cache policy

Looking for ideas on why our Perc 6i won't set the cache policy for writes to "Write Back".

We have two logical volumes (one RAID 1 and one RAID 5) made up of Dell SAS drives.

According to Server Administrator, everything is green (OK). The cache battery shows OK.  The information screen for the Perc 6i shows it has 256MB cache, but for no good reason, the current cache policy is for writes is WRITE THROUGH.  When I choose WRITE BACK, it accepts the request (no error), and even the SA logs show command accepted successfully, yet the write cache policy is always WRITE THROUGH, and our write performance is terrible.

Anyone have any ideas how we can get our Perc 6i to set to WRITE BACK? 

Thanks.