They all have blogs of some sort. Getting clients to pay invoices in a timely manner is a common topic.
If you’re having problems getting your invoices paid, it’s your fault.
Invoices don’t get paid for three reasons, in order of likelihood:
- You didn’t choose your client correctly.
- You didn’t scope the project adequately.
- You are providing a bad product.
If you’re doing freelance software work, you will get approached by people who can’t afford you. Generally, this is a person working alone who “has an idea”. Your goal is to convince them they don’t actually want to do the project.
It’s more expensive than they think, and because they’re inexperienced, there will be more scope changes than a normal project, artificially increasing costs. 
Getting them to pay invoices will be tough because they can’t afford it. Also, you will have a bad conscience because you know they won’t get a positive return on this project.
Scoping the Project
It’s possible your client is experienced, but not in this particular domain. You either over-promised, or more likely, didn’t adequately warn your client about scope changes mid-project.
20% of your job as a freelancer is saving the client from themselves. They will want to add features they don’t need. You have to tell them no. Your project won’t “go bad” because you stop short of the original goal, it will “go bad” because your client will overspend and have buyer’s remorse.
At every step, warn your client about possible cost overruns and added complexities. You will immediately make any potential money lost by wrangling in their ambitions.
Providing a Bad Product
It’s possible your client isn’t paying because they have negative feelings towards you because it’s a bad product. Like almost all high-return tasks for a human, you must trade short-term happiness for long-term. Do your best to either do the rest of the project for free, or find someone senior to assist you with the project.
I’m specifically not suggesting to lower your rate, because that is a slippery slope. Now that your client knows the rate is malleable, in the back of their mind, they will wonder if they have an opportunity to lower it again. It will cause squabbles at every invoice.
 I accepted these projects in the beginning of my career. I believe it hurt my career, but I’m not sure. Sometimes you don’t have an option.