I would put “payment terms” on your invoice next time. I also had a poor experience with one of my clients where I didn’t get paid in a timely fashion. I started to put payment terms on there so that way you can hold them accountable for payment, otherwise it’s literally up to them to pay when they feel like it.
A standard payment term is “Net 30”. This means that the person/company has 30 days from receiving the invoice to pay the bill. Then you can put in the fine print that after 30 days there’s a certain percentage of interest perday or week, or something like that.
Hope this helps!