PARIS—When countries across the West emerged from lockdown in the spring, governments trained legions of investigators to identify and isolate people potentially infected with the coronavirus. The goal was to prevent a resurgence of the pathogen.

Four months later, the systems to find people who might pass on infections, known as contact tracing, are in disarray. Europe and the U.S. are each recording tens of thousands of new daily infections.

In France, Spain and England—nations where cases are now rising quickly—investigators have been interviewing far fewer contacts of infected people than officials expected. In some U.S. states and big cities, investigators aren’t even reaching many people who test positive and those who are reached often don’t disclose their contacts. That has prevented investigators from casting a wide net to stop new infections.

Testing backlogs have delayed potentially infectious contacts going into quarantine, giving them more time to infect others. Mobile apps developed to notify users if they have been near an infected person, so they can identify themselves to contact tracers, haven’t been widely used. And Europeans aren’t scrupulously respecting recommendations to quarantine once they have been identified as contacts of an infected person, officials say.

With many governments unwilling to reimpose sweeping lockdowns, epidemiologists say swift contact tracing is essential to limit the spread of Covid-19 because infected people become contagious before they develop symptoms. That means the contacts of people who have tested positive must be identified and isolated quickly before they unknowingly infect others.