Syringe size and placement?

Just jumping in here to figure out if I’m doing something wrong or it is somewhat normal. So when starting TRT I used a 21g and had a few occasions of violent coughing fit. I tried 25g recently and all seemed well. Even less noticeable during entry into the muscle. But once again I seemed to somehow get a little oil into a capillary or vein. I enter through the upper outside quadrant glute area as advised. I even flex the area to make sure it’s a large muscle before going in. Is it just a random luck of the draw thing or should I be doing something different?