Base cooks before the toppings finish

Affects quality

When the base is done before the toppings, bottom heat is dominating, or the toppings were too thick or dense and needed pre-cooking.

Ask yourself

#1
Bottom heat dominant relative to top heat, common with a very hot steel and weak top element
Generally accepted
A steel's high thermal mass can finish the base well before a moderate top element browns the toppings and melts the cheese fully.
#2
Toppings too thick or dense to finish in the base's cook time
Generally accepted
Some toppings genuinely need more time or heat than the base does.

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

Cooking on a pizza steelStrongly supported
How you would know: The base is properly done and the cheese has melted but not browned, on a steel in a domestic oven.
This is the steel's characteristic imbalance rather than a fault. Fitting a steel solves bottom heat so effectively that top heat becomes the limiting factor, and nothing about the steel addresses the top of the pizza.
Do now: Finish under the grill for 60–120 seconds.
Change next time: Position the steel about 15–18 cm below the grill element and treat the grill finish as part of the method rather than a rescue. Check whether your oven disables the grill above a certain set temperature.

Finish briefly under a broiler, or move to a higher rack for the remainder of the bake

Balance top and bottom heat sources, and pre-cook slow-cooking toppings before assembly

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.