Don't know if I have accidentally changed a setting or something else has changed but I now never get automatically handed over to tower either during departure or arrival
Just to add that Dave's suggestion above applies when you are tuning Tower for takeoff. If before landing you do not get a handoff from Approach to Tower, manually tune Tower frequency and hit Hotkey 1 to request landing clearance. This sometimes happens when you are flying an approach procedure that does not line up with the assigned runway until you are very close to the airport.