I’ve found this is what retros are for. You should address problems after they happen with your team and iterate processes or meetings bi-monthly, monthly, or even more often if you can handle it. Waiting until the beginning of a sprint to go over issues risks starting off the work on a bad note.
YES! Retros are important for quickly identifying issues that have already cropped up.
Premortems help you envision a world where the project turned into a dumpster fire. What you come up with during a premortem helps ease conversations when trouble rears its head.
For instance, in the premortem, we identified one possible problem as "Cadillac taste on a Chevy budget". During the project, the client started to demand custom work that was not in the scope. When talking with the client we reminded them that failure by running out of money is something that we'd like to avoid.