Mostly it's the lower price point, made in China ones, or thin ones that don't have thick metal bases. But, I've even seen diamond plates with a thick metal base that were not flat -- indeed, I even had a DMT that wasn't perfectly flat, though I'm sure it was flat enough for most people's use.
I'm in Japan now, so some of the brands available are different from what's in the US and other parts of the world, but I frequently check the reviews on Amazon, for example, and find people complaining about this or that plate not being flat. I always just buy my diamond plates in person now and check them with a straight edge (if the packaging or store allows). Unfortunately I don't know which brand is safe, and some brands appear to be a little bit hit or miss. If anyone else knows, I'd love to hear also.
I recently got a Kanaban and some loose silicon carbide to see if it might partially substitute for my coarse diamond stones when doing things like flattening the backs of chisels, plane irons, and sharpening stones (as all of these activities have worn out expensive diamond plates in my experience)