The usual cause IF you are using lazy loading and function form import statements is importing the routing module instead of the page module. So:
Incorrect:
loadChildren: () => import('./../home-routing.module').then(m => m.HomePageRoutingModule)
Correct:
loadChildren: () => import('./../home.module').then(m => m.HomePageModule)
You might get away with this for a while, but eventually it will cause problems.