A firm has consistently adjusted its allowance account at the end of the fiscal year by adding a fixed percent of the period's sales on account. After seven years, the balance in Allowance for...

A firm has consistently adjusted its allowance account at the end of the fiscal year by adding a fixed percent of the period's sales on account. After seven years, the balance in Allowance for Doubtful Accounts has become very large in relation to the balance in Accounts Receivable.  Please give two possible explanations.