Large print beds are bad about warping. It's a very common problem and gets worse as the bed gets bigger. The simplest solution I'm aware of is using a thick piece of glass that's extra rigid; only clip the glass down on one side. Clipping the glass down on multiple sides just bends it to conform to your already warped heat bed. You'll know you're doing it right if you can see a slight gap between the glass and pcb in several areas. You lose efficient/effective heating but gain a perfectly level build platform. This has worked well for me and it's definitely the simplest/cheapest solution.