How does this compare to your ability to think of major research questions in various subfields? It's possible that it's just harder to keep up with current research, either because keeping current is always hard or because there's more stuff you have to know now compared to the past. The examples I hear the most about in physics are models for particle physics beyond the standard model, macroscopic models of gravity and dark energy, and the gigantic muddle over how high temperature superconductivity works.