I have a mesh which I fixed up by joining the vertices but when where were joined, the mesh at the section which is circled became flat rather than having its initial curve. When I add the matcap red colour its obvious it had shading issue. How can I get that section which looks flat to become more curvy so it matches the rest of the mesh?
The file you share is not what you show, but anyway what you should do is simplify your mesh to easily give it corrections. To do it you can 1) select edge rings 2) checker deselect 3) select the loops 4) dissolve edges. Then it will be much easier to correct. You could also try the Shrinkwrap modifier to make your suface stick to a round surface.
