Often, that's the hardest part. Most of the time, the root cause is hidden. It takes some detective work to dig it out.
One of the simplest techniques is 5 why. This originated from an oriental philosopher (who's name I have long forgotten) who basically said if you ask why enough times you can find the cause to any problem. Often, 5 is the magic number, but not always. It could take more or less.
Something to pay attention to, is that you may not be the best person in the organization to solve the problem. The standard alludes to it being the responsibility of the manager over the area where the nonconformance was found. It lease it specifically says so for internal audit nonconformances. If you are the management representative, your role should be to make sure it is responded to, and, if needed, be a coach or cheerleader to aid in getting it done. If that is not how it works in your organization, I suggest you have a talk with top management and try to help them to understand that ISO never intended for all quality matters to be the responsibility of the management representative.
Assuming that you can't shift the responsibility to where it belongs, enlist the help of the appropriate manager to help find the root cause. Sometimes it is also useful to enlist the help of the person working where the nonconformance is found. After all, they are the true expert. A wise mentor of mine once said the expert of any job is the person doing it, and as soon as you move as little as three feet away, you are no longer the expert.