At f/3.5, your problem is probably shallow depth of field combined with being slightly out of focus. This is especially likely if the subject is moving towards or away from you.

Other sources of blur include motion blur and poor quality.

Motion blur comes from two sources: camera-shake, and subject movement. At your shutter speed and lens length, camera shake is most likely not the problem. Using a tripod can reduce camera-shake, but won't help with the blur caused by a moving subject. A higher shutter speed, and panning with the subject can alleviate this problem.

If the problem is subject movement, then whatever is moving fastest in the photo will show the most blur.

If no part of the image is sharp, or the wrong part is sharp, then the problem is most likely shallow depth of field and misplaced focus

Motion blur and shallow depth of field also look different. Compare your photo with examples of motion blur and shallow depth of field and you will be able to tell which it is, and if you can't tell, post a photo and we can help.

Finally, poor image quality may make an image not as sharp as you wish. This may be caused by using cheap filters, having your ISO set so high that the image is very noisy, having a dirty front element on your lens, poor lighting conditions, etc.