You didn’t get your hanging rails plumb and square. Remove the cabinet. Redo the hanging rails to be plumb and square, using shims where needed. When corners are substantially out of square and more than 90, the IKEA system has a big problem, because you need big shims, and the rail needs to be 100% supported. If the walls are less than 90, it’s an even bigger problem.

Do a 3-4-5 on all your corners before you start. Figure out which wall is truer. Start there.

If one or both walls are out of plumb, you'll never get the cabinet correct without shimming the rails to compensate. Remove the cabinet, take a longer level (4ft for instance) and place it vertically on each wall. Check if they are out at the top or bottom or even if there's a bow in the wall. Do this in the corner and out the depth of the corner cabinet. Shim the rails so that when you put the level on the wall again with the rails, they both show plumb. If the wall is out at the bottom or bowed, you will need to shim at the bottom of the cabinet when you hang it & not the rail.

Unfortunately, as GreenDesigns pointed out, the rail system is not well suited for this. Without it being 100% against the wall, it could bend/warp.

If you're not familiar with the 3-4-5, here's what we mean.

Measure out from the wall corner 3 feet, or any multiple of it, on one wall. Measure out from the corner on the opposite wall 4 feet or the same multiple as the 3 foot. Both should be measured at the same level plane. Mark each point. Tape with a pen/pencil mark is fine. Now measure across from the 3 foot to the 4 foot markings. That distance should be 5 feet or the same multiple as the 3 & 4 foot markings. In other words, 3-4-5 could be 6-8-10, 12-16-20, etc.

If the measurement is less than 5 feet, the corner is less the 90 degrees. If the measurement is greater than 5 feet, the corner is greater than 90 degrees.

3 4 5 rule explained