Cheese does not brown

Cosmetic

Usually a cheese-moisture-to-bake-time mismatch, not a fault with the cheese itself.

Ask yourself

#1
High-moisture cheese used in a short bake
Strongly supported
A high-moisture cheese like fresh mozzarella needs more time for surface water to evaporate before it can brown, which a very short bake may not allow.
#2
Insufficient top heat
Generally accepted
Some ovens simply lack enough top-element intensity to brown cheese within a reasonable bake time.

The same symptom has different answers on different equipment. Each branch below states how to tell whether it is yours.

Pizza in a domestic ovenStrongly supported
How you would know: The cheese has melted evenly and stayed white, and the base is fully cooked, in a domestic oven.
The cheese never reached browning temperature. A domestic oven's convected heat at 250 °C melts cheese comfortably but does not deliver enough radiant energy to its surface to brown it in the available time.
Do now: Grill/broil for 60–90 seconds.
Change next time: Use low-moisture rather than fresh mozzarella for domestic-oven bakes, position the surface higher in the oven, and finish under the grill as standard.

Finish briefly under a broiler if the base is already done

Choose a lower-moisture cheese if browning matters, or extend bake time slightly with a slightly lower temperature

Guides that treat this as an outcome to improve, an environment to work with, or a flavour to understand, rather than only a problem to fix.