Why do atomic clocks occasionally fail? Atomic clocks like the NIST-F1 are extremely accurate yet they can lose a second in a time frame of several million years. Why aren't atomic clocks 100% accurate?
 A: Atomic clocks are not perfectly accurate due to various systematic effects that slightly modify the atomic transition frequency from the "unperturbed" frequency. A good example is a drifting magnetic field. The presence of a magnetic field changes the transition frequency. Often atomic clocks may be operated at a static non-zero magnetic field, but the magnitude of this field can be measured and accounted for occasionally. But, if the field drifts over time, this can result in uncompensated drifts in the transition frequency and thus uncompensated drifts in the counted time.
An analogy to a mechanical clock is the following: If you put a pendulum clock on a boat the rocking of the boat will jiggle the pendulum causing it to keep time wrong. In the atomic case the magnetic field jiggles the atomic transition frequency.
Here is a table tabulating some of the systematic uncertainties in an optical Al+ ion clock, one of the most accurate clocks in the world. Brewer et. al. (2019)

I'll break a few of these down to explain:

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*In an ion Paul trap the ions are trapped using DC and RF electric fields. Typically ions are trapped at the nulls of the RF field so that they experience no RF electric field. However, the ions aren't typically EXACTLY at the nulls for a number or reasons including thermal excitation. This means the ions experience a periodic driving force which causes the ions to oscillate. This motion results in Doppler shifts when interrogating the ions with lasers for frequency readout which can result in frequency shifts.

*Blackbody radiation: The experiment outside the ions (i.e. the vacuum chamber and the lab outside) are at room temperature. This means these surfaces emit some blackbody IR radiation onto the ions. This radiation shifts the clock frequency via AC Stark shifts. If the temperature or geometry etc. aren't known perfectly this shift can't be exactly calculated and compensated.

*Quadratic Zeeman: This is a clock shift due to unknown excess magnetic field as described above.

