Depth of field gets very shallow at close focusing distances, so this is normal in macro photography. To increase it, stop the lens down to a smaller aperture such as f/16 or similar, which will give you more depth of field than shooting wide open at f/2.8. Keep ISO as low as practical for best image quality, and add more light if needed so you can use that smaller aperture.

Because less light reaches the sensor at small apertures, you may need a slower shutter speed, but make sure it is still fast enough to avoid blur from camera shake or subject movement.

Another simple option is to back up slightly so you’re not shooting at the closest possible distance, since depth of field increases as magnification decreases. You can then crop the image afterward if needed.

In macro, though, depth of field will always be limited, so combining a smaller aperture with good lighting and careful shooting distance is usually the best approach.