There's a good chance this is unfixable I'm afraid*.
The likely causes are the wood itself (HD wood being generally of mediocre or poor quality, although standards apparently vary widely across the country), a minor discrepancy in the corner joint or in the way the pocket screws were drilled for or driven home.
In the first case there's no way to fix it reliably, not in a completed frame. You can try bending the piece back to flat on the workbench using clamp pressure but that nearly never works, and it stresses glue joints so will weaken them (not just the joint at the high corner but possibly all through the structure) which is bad news for long-term stability.
In the second and third cases unless you break the joint apart and try to fix the direct cause the joint as it exists will do its best to keep the shape as it is.
*It's usually best when something like this occurs to take the thing apart and earmark the wood for future projects. And once you have it apart if the wood itself was the cause you'll see it immediately, as one or more of the boards (my money is on the stile) won't be sitting flat like it did prior to assembly.