Hi Lee. That was my guess too, but thought I would ask.
I remember that incident with Dave Sheldrake and his stock of stuff was pretty bad. He couldnt tell from sheet to sheet what the settings were going to be. Unfortunately he had a large amount of stock on hand for his business. I assume he has worked that out since.
I tend to buy what I need as I go. An advantage to living just a few miles from good sources of supply. I also spray all my sheet stock with either lacquer or clear shellac as soon as I get it home using an HVLP sprayer so even if humidity is changing it affects my stock more slowly. And the coating also makes it easy to deal with any soot if my settings are not spot on.
Dave
900x600 80watt EFR Tube laser from Liaocheng Ray Fine Tech LTD. Also a 900x600 2.5kw spindle CNC from Ray Fine. And my main tool, a well used and loved Jet 1642 Woodlathe with an outboard toolrest that helps me work from 36 inch diameters down to reallllllly tiny stuff.