The Russians used to say "they pretend to pay me so I pretend to work".
Funny thing how some start-up managers think everyone should underwrite their dream, most especially their service providers. The phone company will cut them off after a couple of month and is righteous about imposing and collecting late payment penalties; so is the credit card company. Landlords will toss them out of their space. If you don't pay your hosting service things get sticky after a while. See what happens when you try not pay the guy at the food truck or the pizza place.
When one of my clients is tight on cash we talk to our vendors, make partial payments and always return their phone calls. If we've committed to paying on a certain schedule and realize we're going to be behind by more than a few days we let them know and ask for their cooperation. It's amazing how much help and consideration you can get if you demonstrate good intention.
The flipside is that if someone goes dark, we reach out to find out what's going on. If we don't hear back we slow down and eventually stop delivering. As long as they're behind they are a low priority client. I will be polite (until they've made it clear they don't care about paying) but we can always decide whose call gets answered first and how long we talk to them.
A friend of mine told me years ago "I don't take risk to break even. If I take risk I expect to be paid well for it." A client who pays slow or sporadically should not only be paying penalties for not keeping their word but should be charged a higher base for the annoyance, distraction and disruption they will cause to your business.
Put bluntly, your client hopes the work you are doing is going to make them very rich. If they don't keep their promises to you and can't find the time or energy to at least keep in touch, you have no obligation to help make them rich. Your first obligation is take care of your family, your team and yourself.