yes, get it quite often..... my first tactic is to re-send the invoice, just in case it didn't go through the first time. Following which I'll re-send it again with a direct note to say it's overdue please could they chase it up for me. After that... thankfully hasn't happened yet, that usually does the trick and fingers crossed it stays that way!
Hope you get it sorted
And yes, it is frustrating when you rely on the money...