Your problem could be one of many or a combination of many. You start to lose money if your population gets too high, you max out the population's taxes and they produce less, your army is too large so that their salaries and upkeep is more than your income, your trade is disrupted by the enemy in your territory or blockading your ports. I got a lot of reasons, but I'm sure other people can name even more that I probably don't know about.
I put a dollar in one of those change machines. Nothing changed. ~George Carlin