When you have other writer friends, belly aching about checks and invoicing is a common theme on social media. Sometimes we discuss ways to collect monies owed, because being paid in a timely manner is thing as a freelancer.
There are a million reasons for this. Sometimes it's the writer's fault for not invoicing correctly or not getting a contract that is clear on a payment time frame, and sometimes it's the other end that dropped the ball. Either way, it's an area that everyone can improve in.
And sometimes I have to white out commenters on a thread, because they are a much bigger deal than I am. :-)