What do you do if a publisher stops paying?

I've been an affiliate for a publisher that is using tracking platform (like comissionjunction) since November. This publisher pays via check at the end of the month for the previous month (October's earnings are paid at the end of November.) I received my first check on time at the end of December, which was about $150. The second check was a bit of an issue, I had to contact the publisher multiple times before a check was cut. I received the check for Decembers earnings about 3-4 weeks late, in the middle of February. This check was for roughly $350.

January's earnings (roughly $750) haven't been paid yet, and it's almost time for February's earnings (roughly $650) to be paid as well. I've tried contacting the publisher multiple times through email and have received no response. I've left multiple voice mails as well with no response. I've stopped promoting the publisher on my website and switched to a different publisher and a different tracking platform. I can't seem to find any support from the tracking platform for affiliates. I've emailed the main contact email for them asking who to speak to. I've also tried calling the main number but I never get through to a person. It either goes to a voice mail that's full, or says that the extension is unavailable at this time.

If this was $100 or less, I'd probably just chalk it up to a learning experience and move on. However, with unpaid earnings from Jan/Feb and this month until I stopped, it's over $1,500. I'm not really sure how to proceed with this.

If anyone has had a somewhat similar experience with publishers not paying, would you mind sharing how you went about dealing with it? Any help is greatly appreciated.

TL;DR Publisher hasn't paid in two months, my earnings are over $1,500. Can't get in contact with the publisher at all, or the tracking platform. Not sure what to do.