I set up all my bills to pay thru ACH. They all go thru for the most part. However, I had 2 separate situations where my water bill got rejected. It would go thru one month and for some reason not go thru the next. The rejection code is bad account number. However, nothing changed. It was the same account number I originally set up the ACH payments with. My bank says they didn't reject it but the Water company says the bank denied it due to bad account number. Meanwhile I'm stuck with a fee. There was plenty of money in the account each time. What causes these rejections and how can I avoid them?