Hi scubamom,
Another possibility is that the steel frame you are using is not supporting the PLB foundations correctly. I have a PLB queen size Nature mattress with a PLB matching foundation. I was not pleased with the construction of their foundation. See Post #5
We do not have any sags in our mattress but a few months ago the foundation started to creak. I called them a few weeks ago and could not reach them but left a voice message and they got back to me and replaced the foundation. See Post #1 PLB is down but not out yet. They will still honor your warranty.
My advice is get the PLB mattress replaced under the warranty if the impressions are 3/4" or more deep. Do not use the PLB foundations. Get a different foundation (see the foundation thread), and a heavy duty steel frame (if you do not have one); or place the mattress on a platform bed.