The problem all began when I bought large canvases (I'm talking 3 feet by 4 feet and whatnot) and stored them in the garage.
Do not store your large canvases at even the slightest angle in your garage. Because of the temperature and humidity changes it will warp the frame.
You end up with one side of your painting looking like it thinks the wall has cooties.
From the front it looks fine. Although, with a light coming down on it you get an unsightly shadow.
Here is a solution that I had to find scouring the art forums:
1. Lay your painting face side down in an area people won't be walking over or tripping on it. (I did it in my garage).
2. Spray all wood with water.
3. Weigh down the corners with random heavy objects in your garage. (For example, I used two loaded picnic baskets, laundry detergent, and a backpack of my husband's with music stuff--and it's surprisingly heavy).
4. Leave overnight or for however much time you see fit.
5. Hang up on the wall and be ecstatic that your $80 canvas is like new!
Disclaimer: it might not be perfectly straight. But if all the corners lay flat I call that a SUCCESS!