I'm having difficulty getting DNS to autocreate PTR records.

I first created the subnet,, then created several A records
beneath. I looked around here when it wasn't auto-creating, and realized
I needed to create the 25.20.172-IN-ADDR-ARPA record, so I did. Then I
created another A record beneath the for .84 (one of our
printers). It didn't autocreate the PTR in the IN-ADDR zone like I
thought it would. I looked at as many options as I could on both zone
records and don't see anything out of whack. Any ideas?