[Army of One]

Get a good debt collector.

Give a final warning letter and follow up call / message + email saying payment must be made by XX date or it will be referred to a debt collector. Ensure you follow through with it.

I know this is difficult - but don't let it shake your confidence. I can sympathise and know how frustrating it is when people do not communicate with you!

Other ideas moving forward:
Maybe also have your 'terms of trade' looked at and updated if necessary - ensure it is all incorporated as part of your quoting process.

Have you looked a fee payment service where you get paid upfront but your clients pay it off over 3,6 or 12 months? We sometimes utilise Smartfee which provides this service in our industry - but there are others.

Hope it turns out for the best (for you - not for your them!)