A factoring company has a limit on financing any invoice beyond ninety days. In other words accounts receivable financing is only for businesses that issue thirty day net term invoices and have a history of getting paid on time. If the cash flow model of your business is not getting payment for work completed within a relatively short time (vs. quarterly or annually) then factoring invoices might not be a solution for you.
In further detail, the reason is, the arrangement a factor has with its funding source usually is a leveraged institutional bank line. The bank lends money to the factor who then turns around and makes advances to their clients. When calculating the available credit to the factor, any outstanding invoice over ninety days is considered a non-performing asset and is deducted from the available credit line.
In some cases an invoice does go over ninety days. In these situations a recently finished job with a new invoice is swapped for the overdue invoice. But this should be on rare occasions and not the norm.
So make sure when invoice factoring is an option that the receivables are being paid in a timely fashion.