Under Network > Provisioning > Logging, enable the dynamic IP logging, it will give you more details. You can also run the script that you put into the DHCP server by hand with a specific IP/MAC directly in the MikroTik command line interface and see what you get back.
95% of the time, it's a bad username/password for the API, or the MAC/IP is unknown in Sonar. Running the script by hand will normally tell you that. As long as it's not a bad username/password, turning on the logging will diagnose any other problems.