Sounds like a case ripe for litigation. Do not be afraid to assert your claim. In most states loser pays the court filing fees in addition to damages. They may not be responding to your emails, etc., but a court summons duly served will jolt them out of this pattern, or they will lose by default. In which case you may not even need to appear in court depending upon the procedures of the court where you file.

In most cases sending them a copy of the summons and complaint prior to filing with the court is enough to spark a response from slow paying clients. If they fail to respond to it...file it with the court and serve them with the papers.

I had a fun experience several years ago with a party located in another state. I did not get paid for some work I had done. I  called to make inquiry about it. The indivudal to whom I spoke was rather arrogant, and indicated that he had more important matters to attend to. I informed him that under Connecticut law attorney's have a possessory lien on all client files until paid in full. The lien travels with the file, and all who take possession of the file and profit thereby become subject to the lien, and its foreclosure. Ultimately I would garnish the mortgagor's monthly mortgage payment to the lender..

In Connecticut you can not sue a non-resident defendant in small claims unless he has real estate located in Connecticut, but you can sue them in superior court under our long arm statute (more expensive costs for the defendant to pay).Even if the defendant chose to challenge the court's long arm jurisdiction it would have been far more expensive than paying my bill.  I explained the lien on the file to this gentleman, and told him that both he and his client could come to Connecticut to explain their conduct to a judge. He made arrangements to pay my bill rather quickly.