Superfrost or Superfrost Plus? That’s a big difference and it’s happened before that I use just Superfrost (by accident) and then have all my tissue fall off. Check to see if there are two plus marks on the side opposite the white frosted area.
Try 12 um. For PBS rinses, nothing special. I use this easy dip system (https://www.emsdiasum.com/microscopy/products/histology/staining.aspx) and just take the slide rack out, replace the PBS, and then put back in.
Have you done much DRG slices/IHC before? Do you always have this issue, or this is your first time? And what do other people in your lab do? Like I said, this happens only very rarely for me.
If you are using Superfrost Plus, maybe try a new batch?