Put a payment term at the bottom of your invoices of 28days if just domestic then send a copy of the invoice with a reminder cover note but do not word it negatively and print you contact details with a note saying 'Don't hesitate to contact you if you wish to discuss the above invoice'.
Give it another 2-weeks and make direct contact either by phone or go to see them, again be polite, ask if there is any issue or problem paying your bill that could be resolved with a 5min chat... ask for the bill to be settled withi the next 7 days and express they are 21 - days overdue already -- end the conversation on good terms do not get demanding.
If no payment within 10-days then send a final demand for immediate payment with a sterner worded letter, call them 48hrs later and express you are passing the owed money onto a 3rd party 'Debt collecting agency' and they will pay yourself in full and seek to collect the money + additional costs for their time and services 'typically a few hundred quid for the first visit'
This usually is what gets the truth (which cant be discussed) or a immediate payment.
NOTE!!! this is for domestic only ... commercial and industrial customers work differently and you need to discuss their payment terms before taking on the work, either make an arrangement beforehand or even discuss a 5% / 10% discount for early payment.. many of my larger customers pay on 8 weeks but some pay on 12 weeks, this is not uncommon and you need to ensure you can ride this payment term as pestering accounts for them to change their payment terms just for you can have negative impact on future works.