If you are doing work for a very large company or even a U.S. Federal government agency, but you actually invoice another company, they are the PRIME contractor who then invoices the large end user entity. That makes you the SUB contractor. Even though the end user is a large organization and probably creditworthy, when factoring invoices it’s the actual payor that has to be have a solid credit history. This may create an issue when considering the creditworthiness of the account debtor. A factoring company only can consider the credit availability of the actual company making the direct payment.
Many large companies create a preferred vendor list. Any new vendor that wants to do business with that large end user has to go through the preferred vendor. It makes life more efficient for the end user because they only have to keep track of a few vendors. But if you end up as the sub contractor in these occasions it places a layer between you and getting paid. You may be at the mercy of the preferred vendor.
The best thing you can do is dilute the risk by continuing selling your services so that the amount of your sub contract invoices is not the majority of your total outstanding account receivables. Any time your invoices are considered high risk you will have trouble securing credit against them.