It's your company. You set the terms you're willing to work for. Do you think these companies don't have the money or are just being jerks and being late? Can you afford to lose these clients? I'd start off by telling them that they are going to need to have payment for the oldest job and the current job before you start doing ANYTHING on the current job. If this doesn't work for them I imagine there are bigger issues at hand.

I am not in this type of business. Are you friends with your competitors? In my professional circle we have a naughty list of deadbeat clients and are very happy to privately share details of people who pay late or not at all.