Why does frost keep building up in my freezer?

Humidity is the local variable. Every time the door opens on a house near the water, the cabinet takes in air with more moisture in it than the same freezer would see in an inland valley, and all of that moisture has to leave again through the defrost cycle. A defrost system that is marginal rather than broken keeps up in a dry climate and falls behind here. That is why the same model, same age, fails this way more often on the coast, and why a gasket is a genuine repair on a freezer rather than a cosmetic one.

The condenser is involved more often than people expect. A coil loaded with salt and dust rejects heat poorly, so the compressor runs longer to hold the same temperature. Defrost intervals are built around normal run time, and once run time climbs the cycle no longer arrives often enough to clear what has accumulated. The freezer then ices progressively rather than failing outright, which is exactly why owners describe it as getting slowly worse over months instead of breaking on a particular day.

What we do about it. The evaporator cover comes off and the coil gets a proper look, because the ice pattern tells you which of the three faults you have. Uniform frost across the whole coil points one way, a solid block at the bottom near the drain points another. Then continuity on the defrost heater, resistance on the thermistor, and the drain checked and flushed while the cover is off. The gasket is measured with the door closed, all the way around. Clearing the ice without finding the cause simply resets the clock on the same visit happening again.

If that matches what your appliance is doing, say so when you call — it tells us which parts to bring, and most of these are finished in one visit.