"Kateřina does not like him."
Translation:Kateřina ho nemá ráda.
In the second position we usually use the shorter clitic form ho. You can use jeho in other positions.
Kateřina mená ráda jeho. Jeho Kateřina ráda nemá. ...
Both examples above strongly stress jeho.
What stresses him more, ho on the second position or jeho on other positions?
I would expect that "jeho" would be fine. Maybe there was something else that DL didn't like in your answer. It's usually helpful to paste your answer into your comment, so that the people behind the scenes can see the whole translation, and tell you exactly what's not quite right.