Diagnosis
Sun and heat. The ABS plastic lid can warp if it gets hot from the sun while secured tightly over the cool griddle top, creating a temperature differential and vacuum stress. It's a common issue with aftermarket hard covers stored outdoors.
How to Fix It
Once warped, it's hard to permanently fix plastic. You can try laying it flat in the sun (outside of the griddle) with some weight on it to gently flatten it, but results vary. The best fix is prevention: store the lid off the griddle in a shaded area, or use the official Blackstone soft cover over the whole unit if leaving outside.
If That Didn't Work
If it's only slightly warped, you might still get it to latch by pressing down firmly on the high spots while engaging the latches. Don't force it, though—you can break the latches.
Heads Up
The hard cover is mainly for transport and short-term protection. For long-term outdoor storage, a fitted, vented soft cover is a better choice because it doesn't create a tight seal that can trap heat and moisture, which leads to rust and warping.