The trick is to pose your subject and compose the shot in a manner that allows you to have a pleasing crop of the subject AND still keep interesting parts of the photos on the thirds. It just takes practice. You don’t need to sacrifice one for the other, but you may have to think for a second how best to satisfy both objectives.
While both errors are poor technique, for me personally, it is more distracting to have bad crops of body parts than it is to have someone a little off one of the lines. But thats just me. You have a little wiggle room to work with on the rule of thirds, but really, as long as you avoid bulls-eyeing a persons face in the center of the frame and make sure your horizons are generally on the lower or upper third, while making good crops of body parts, you’ll do fine.
Usually one of these two mistakes causes the other mistake to happen, or make it worse if its already present. In other words, when someone is poorly cropped, it will make it even more difficult to get a good composition in regards to the Rule of Thirds, and likewise, when a subject is poorly positioned in the frame (tons of headspace for example), its almost always going to result in a bad crop of the body or body parts. Fixing one usually helps make the other better or at least easier to achieve.
The photos in the session are improved from the last set I saw of yours in another thread. I can tell you are thinking about composition and placement. The thorn is that some aspects are met in many of the photos, but one or two slip by you, and the technique error changes from photo to photo. Some photos have good adherence to Rule of Thirds, but her foot or hand is cut off at the joint. Other photos have her pose looking fine, but her face is square in the center of the frame (sitting against a tree, for example). On one photo where your subject is lying on her stomach in the grass and you are shooting from a low perspective, be mindful of body parts pointing into the camera. Her arm is extending towards the lens in one shot that would otherwise be quite pleasant. Body parts pointing into the camera are usually distracting.
Keep shooting! You’re getting better and I’m glad you want to improve.