My experience is that there’s “flat” then there’s FLAT . A very common gotcha is that even a visually undetectable warping of the material can cause issues. If you tap anywhere on your material and hear a click or an hollow sound, it’s not FLAT .
Your best bet is to cut out a dozen or so Honeycomb Bed Holddown Pins and use them to make absolutely sure your material is firmly secured. I cut out a few from draftboard scraps leftover from various jobs – you cannot have too many of these. 
My cutting success skyrocketed when I adopted these as part of my workflow.
Best of luck!