If you want to know how to motivate a person, start by asking them. Maybe they will be self aware enough to identify what it is, maybe not. If they don’t know what motivates them, ask them to take some time over the next week to reflect on a time where they were motivated at their job, and you can discuss it further in your next one on one. Then, take that scenario and start to deconstruct it. What was it about this time that was different than other situations? How long did that motivation last? Who were they working with at that time, and how much did the people play a role in keeping you motivated? How much do factors like stress and salary impact motivation for them?
Motivation is also fluid. What motivates someone at 25 is different than at 35. At 25 I craved recognition for having potential. At 35 I crave autonomy. Some fundamental components haven’t changed, but the overall story has.
So, when in doubt, and even when not in doubt, start by asking.