If you're lucky, most of your students will be motivated, focused, and excited to come to lessons. Even the most engaged students, though, will inevitably run up against something that they just don't want to work on. This usually rears its head during etude work; it's just not as much fun to grind away at technical studies.
When your student tells you they don't want to work on something, don't just leave it at that. Ask some questions. See if you can determine where the resistance is coming from.
I remember back in high school, my flute teacher had me work on a book of Andersen etudes that I just hated. I found them frustrating because I was used to things coming easily to me, and these etudes were a challenge. I finally threw my hands up in the air during one of my lessons and told my teacher, "I just don't see what I'm supposed to be getting out of this."
That statement was totally true and totally valid. But that sentiment in no way meant that I shouldn't have been working on those etudes. Part of taking lessons is finding out what you didn't know you needed to be doing. And while I needed those etudes to take me to the next level, I wasn't a developed enough musician yet to see that.
Now, when I teach my own students, I make sure that they know what they'll be gaining from what we work on together, especially if it's something that might make them feel stupid or unskilled for not being able to master right away.
So if you experience pushback from a student over something that you know will be good for them, sit and have a chat with them. Listen to what they say and help them understand. And at the end of the day, some things just need to be put on the shelf for another year.