Make the problem explain itself through testing.

Arguing about what is wrong can go on indefinitely, because opinions do not resolve each other. A test does. Setting up a small experiment where the situation has to reveal something usually settles in an afternoon what discussion could not settle in a fortnight, and it replaces confident guesses with something everyone can see.

When you are unsure what is happening, ask what small thing you could try that would produce different results depending on which explanation is right. Then do that rather than thinking harder. The problem will often tell you plainly once you ask it a question it has to answer.