I’m sorry that you’re still having trouble with veneer. Could you measure your material with a pair of precision calipers (or another precise measuring tool) if you have them, and send me the values you record? Be sure to remove the protective layer on both sides before measuring.
Follow the link here for a very useful item, the Honeycomb Hold Down Pins. For best results, the material needs to be as flat as possible. Using these will assist with holding down any item that you print on.