One example is that you need a rare expert to do something and there are no extra unemployed experts available to you, but you have more work.

If you make the ones you have more productive, you don't reduce employment.

It is also true if there is a large fixed cost per employed person. For example, a very expensive machine.

So, you have work for another person, but can't afford the extra machine. Making the people you have more productive also does not reduce employment. It could increase employment if profit per person makes the machine more affordable, and then you can hire someone.