Customer payments get delayed all the time. It may not always be worrisome right away, but not having an accounts receivables strategy can be devastating for your business.
Consumers and businesses alike have ups and downs, and their financial situation can temporarily or permanently deteriorate. If you have outstanding AR from your customers, it is crucial to get paid on time before they prioritize other payments over yours.
Firstly, you must learn to overcome the hesitation of reminding your customer of a missed payment promptly. It is possible that it was just an oversight on their part, but there is always a possibility that they have a financial crunch. They could be owing other bills just like yours.
Secondly, there is nothing wrong with sending invoices twice a month and a phone call to the concerned person will ensure that you have done everything to maintain a healthy AR. We prefer emailing the invoice, followed by physically mailing the same invoice ten days later, followed by a phone call to the concerned person after another ten days. It means in 1 month you would have contacted them thrice (by email, mail, and phone call). The process should repeat for at least 2-3 months until you get paid.
During this time, you must assess your dependency on this customer and you need to prepare for an alternate strategy. If your invoice is between 90-120 days old, I do not doubt that most of these accounts should be forwarded to a collection agency.