You need to go to your project security > user roles and make sure correct module user role is selected for each role. Than go to module security > Page access, Nanoflow access, Microflow access tabs appropriately to select or re-select module role for individual page, nanoflow and/or microflow.
Not sure if you already went though security learning paths in Mendix academy? if not than Mendix academy has got some learning paths on security and are beneficial. This will be the good one to start with!
Hope this helps!