Those excuses are just there to make you wait to get paid. Have found that 95% of the time they are bogus - especally when you have been paid for other work by that company.
For those type of customers I raise rates as soon as I can.
I also charge late payment fees, after all if you are late to your supply house they will charge you.
I know that many of those companies do not pay the late fees but it does seem to get them to pay on time on the next billing.
