Why Is My Online Payment Sometimes Accepted And Sometimes Not?

I paid my last bill online on 7/31/12, and got confirmation #2380574.  To me, a confirmation # means CONFIRMED!! A few days letter I get an email that it wasn't actually confirmed, and then a letter saying I owed 2 months payment!!   I have used the same bank and the same account and routing numbers every time I try to pay online, but sometimes it goes through and sometimes it doesn't!! I am living on unemployment, and I can't affodr to pay 2 months premium at a time.  I also can't afford not to have the payment accepted when I make it, because by the time I get notice that it hasn't been accepted for some reason, I'm out of money!   PLEASE tell me how my online payment can work right EVERYtime.   Kay Bartosz