If you've tried the fixes above and still get weak foam, check these.

Is your matcha actually ceremonial grade? Culinary grade won't foam as well and tastes more bitter. Check the package or supplier—look for 'ceremonial' on the label.

Is your water too hard? High mineral content can interfere with foam formation. Try filtered or bottled water if tap water is hard.

Is your whisk old? Bamboo chasens lose tines over time. If yours is bent or missing bristles, replace it—it's a cheap fix.