This is the defining senior skill, so it deserves a method rather than advice.
A test fails on the build server. You run it locally a hundred times and it passes every time. You have no reproduction, and the machine it failed on no longer exists. Everything you get is what was recorded.
Start by classifying the failure. The class determines the investigation.
Class one: it fails alone, consistently, in CI only. Something about the environment differs. This is the most tractable class.
Class two: it passes alone, fails in the suite. Another test is affecting it. Nothing about the environment is relevant.