Not Enough Units: Do you have enough gold to buy additional units during the game? You can purchase new units and deploy mid-game if need be. I had to do that once or twice. OR don't let your units dwindle down. Get to a city and buy recruits more often.
Time: The majority of my victories for all 3 campaigns are bronze with a few silver and 1 gold. If you are running out of time try not going for the secondary objectives such as towns that are not required goals.
I am sure this isn't new stuff to anyone but there it is anyway.