Why can't I get consistently sharp images?

Every time I upload a new batch of photos I'm always disappointed because about half of them are not focused on the eyes. I am extremely careful about where I focus and it just seems to be hit or miss. I usually don't shoot wide open, but close to it. I'm pretty sure it's not motion blur. It's just slightly out of focus on the eyes or focused on the nose or something closer to the camera (although I line up the grid and hold it steady right on the closest eyeball). Some turn out beautifully, the rest out of focus. I can't figure out what I'm doing wrong!!! I'm beginning to think it's my camera because every article, set of tips, etc says I should be getting sharp images. Any suggestions?

Below is an example. Every photo in this set was out of focus. Subject was still and I was very careful to hold the camera steady.