You move the camera closer to a flower, but autofocus only hunts back and forth. You may have crossed the minimum focusing distance: the lens cannot form a sharp image at such a short distance. Closer then means neither bigger nor better, simply blurred. A small step backwards is the first useful test before changing settings or equipment.
Where is the distance measured from?
For interchangeable lenses, minimum focusing distance is normally specified from the sensor plane to the subject. It does not begin at the front edge of the lens. Many camera bodies carry a small sensor-plane mark. With a long lens, an apparently generous specification can therefore leave little space in front of the front element.
Working distance instead describes the clear space between the front of the lens and the subject. It matters for lighting and handling. Moving very close can block your own light, introduce camera reflections or touch a flower. More working distance makes it easier to place a reflector or small light source outside the frame.
Zoom and reproduction ratio
Some zoom lenses change their minimum focusing distance with focal length. Check the specification for the zoom position you actually use. A short minimum distance alone also does not reveal how large a small object appears on the sensor. Maximum reproduction ratio is more informative. At 1:1, a ten-millimetre detail is projected ten millimetres wide on the sensor.
A more distant subject photographed at a longer focal length can therefore appear larger than a very close one at wide angle. Compare achievable frame filling rather than centimetre figures alone. Cropping afterwards may be sufficient for a small web image, but it sacrifices pixels and cannot replace captured detail for large output.
Recognize the limit clearly
Test with a stationary high-contrast object in good light. Move the camera slowly backwards until focus becomes reliable. If the problem persists with substantially more distance, investigate other causes such as weak contrast or accidentally enabled manual focus. Do not keep pushing the camera closer: repeated focusing cannot overcome an optical distance limit.
