How do you do when you disagree with UI designer?

I have a little question, I am a full UX Designer, I don't do UI in my company. However, I have a regular problem when I am making user interviews then wireframes. The UI designers of my company said "No we can't do that because this journey is too complicated for us, so, it will obviously be too complicated for our users."

They don't want to create anything after that and they don't want to talk about it. How do you do in this situation ? Are you testing your wireframes ?

Thanks