When employees aren't doing what they want to be doing
Oftentimes, especially on a small team, there will be people who are not doing what they'd like to be doing. It is important to always put the business first yet have a direct conversation about the situation: "Hey Bob, this project is important to the company. I know it's not how you'd like to be spending your time, but it's the most important thing you could be working on, and I'm going to tell you why."
It is important to acknowledge what the person would like to be doing, and if possible, let them know the current misalignment won't last forever. If I don't believe this person will ever get to fulfill their desire to work on what they want, I should also be direct about this.