I am very new to zbrush, what is a skinning modifier? how do I disable a symmetry modifier? Now that I think of it the objects I have that are not working are either not closed objects or they are not symmetrical.
Your popup is unfortunately covering your skinning modifiers so it is hard to tell. If I were to guess, I'd say you have a symmetry modifier enabled on the remesh button. If the model isn't symmetrical across that axis and the half it is trying to mirror over happens to be the half that doesn't have any vertices in it, the…
The skinning modifiers are all the buttons and sliders located around the Remesh All button that alter its behavior (such as the resolution and polish sliders). The Remesh All button itself also has 3 tinier buttons inside of it on the top-right corner (x,y,z) to control the axis of symmetry it uses.