Even new plywood can warp if not stored properly or if left in the sun. Before you give up in frustration, though, try a little moisture therapy.
Like the underside of a shriveling leaf, the plywood’s concave side has lost moisture and shrunk. Reversing the warp can be accomplished by adding moisture to this concave side and drying the convex humped side. I use a sponge or sprayer to wet the concave side of the plywood (hot water works best). Then I lay the sheet, moist side down, on a shop floor or a driveway. Now the sun, or the warm interior air, helps to dry out the convex, humped side of the plywood. At the same time, moisture is being absorbed into the concave side. This process works faster than you can imagine, so keep an eye on the material. If it warps the other way, just reverse the process.
Normé Palmer, Nanjemoy, MD