Have you noticed that on some cardigans, when they are worn buttoned up, that the button band pushes out at every button? If yours do that it may be because you sewed the button on without a shank. Some buttons are sewn on through a loop on the back. This is a shank. This type of button sits a little above the band without distorting it. Others are secured by sewing through holes (usually either two or four). This type of button requires you to create a shank to avoid the distortion. There are a few ways to do this.
You can create a thread shank by placing a double pointed needle between your button and the garment, use a single stitch to first anchor the DPN and then sew the button in place without breaking the yarn, remove the DPN and finish by wrapping the thread around the thread from the garment to the button before securing it. The length of the shank should correspond with the thickness of your knitted fabric so that your button will sit on top without crushing your stitches. Choose your DPN to correspond with the thickness of your knitted fabric and to create a shank of an appropriate length. Not only will you have less distortion, it will also help stop the buttonhole from being stretched out of shape.
There are two other alternatives to create a shank without thread. You can sew a small bead under the button. It's best to choose one that is a good colour match and that has smooth edges that will not break your thread. It also needs to be of a material that will withstand whatever cleaning method you plan to use. The second alternative is to use a small clear button sewn below the garment button. When you sew it on, line up the holes and sew through both buttons at the same time. You could use two buttons if you require a longer shank.