There are many things that a plumber might find complicated, but very few that simply can't be done. The whims of homeowners are many, and sometimes they may want a sink, shower or toilet to be in another place.
If you've been working with the plumber for a while on a renovation, he may not be so speedy to offer up a solution that satisfies you if it makes life more difficult for him. Even though you're paying for it, sometimes it's more cost-effective for him to simply move on to the next job. If your plumber tells you your sink, toilet, or shower can't be moved, ask for a detailed explanation and see if you can't come up with a solution.