Fixing an undermount sink usually means solving one of three problems: the sink has started to loosen, the seal line has failed, or the support underneath is no longer holding the sink securely. The right repair depends on the cause, but the core idea is always the same. The sink needs to be fully supported from below, aligned correctly with the countertop cutout, and sealed with a durable waterproof bond. This matters even more with a quartz sink, because quartz composite models are heavier than many basic metal sinks and need a stronger fixing method.