Stripes and flat, posterised photos have different causes. Mixing them up is why many people change settings for hours without getting closer.
Try Vextrify freeEngrave a plain mid-grey square. If it shows stripes, the cause is the machine or the spacing, not the photo. If the square is even but your photo has no midtones, the cause is the image preparation. This one test saves most of the guesswork.
Check belt tension and that the gantry runs square, then calibrate the scanning offset so lines in both directions meet. Make sure the line interval suits your spot: lines much wider apart than the spot leave visible gaps, lines much closer overlap and darken unevenly. Leave enough overscan so the head reaches full speed before it burns. Change one thing at a time and re-test the grey square.
Materials darken more than a screen suggests, so a photo engraved straight from the camera loses its midtones and its shadows close up. Lift the midtones for the material, keep detail in faces with local contrast rather than global contrast, and compare dot patterns, because some hold smooth gradients better than others. Avoid running a second dot pattern over an already-dithered image.
Photo Studio starts from one of 12 material profiles. For photos on light wood it also separates large shapes from fine detail, so faces keep their lines without turning noisy, and keeps genuinely solid black areas solid while photo areas keep their dot pattern. We tune these steps against a fixed set of three reference photos, so a change that helps one picture cannot quietly hurt the others. You compare ten dithering methods plus grayscale in the live preview before exporting.
Usually mechanical or spacing: loose belts, an uncalibrated scanning offset, too little overscan, or a line interval that does not match the spot size. Test with a plain grey square.
The preparation did not account for how the material darkens, or the dot pattern cannot hold fine steps. Lift the midtones for the material and compare dithering methods.
It depends on the laser and material; compare both on a test piece. On many diode and CO2 setups a dot pattern holds midtones more reliably.